Best Thermal Underwear for Hiking of 2026

Staying warm and dry on a hike starts with the right base layer. The best thermal underwear for hiking keeps your body heat in while letting sweat escape, so you stay comfortable on the trail. But with so many options, choosing can be tricky. In this article, we’ll look at the top thermal underwear for hiking, what to consider before buying, and how to find the perfect fit for your next adventure.

1. REI Co-op Midweight

• Fabric: 92% recycled polyester/8% spandex
• UPF Rating: 50+
• Moisture Wicking: Yes
• Weight: Midweight

The REI Co-op Midweight base layer bottoms are a dependable choice for hiking in cold weather when you need warmth without giving up mobility. Made from a recycled polyester and spandex blend, these thermal bottoms provide a soft, stretchy feel that moves naturally with your legs while hiking over uneven terrain. The midweight construction provides useful insulation for chilly mornings, winter trails and other cold-weather adventures.

Moisture-wicking fabric helps move sweat away from your skin, which can make a noticeable difference during long hikes and changing temperatures. The 50+ UPF rating also provides added protection when trails expose you to strong sunlight. With a fly opening and available 28- and 31-inch inseams, these bottoms offer practical features for layering underneath hiking pants. They are especially well suited to hikers who want a versatile synthetic base layer for regular outdoor use.

Pros:

• Soft and stretchy for comfortable movement
• Midweight warmth for cold-weather hiking
• Moisture-wicking and UPF 50+ protection

Cons:

• May feel warm during high-output hiking
• Synthetic fabric may not appeal to merino wool fans

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2. WEERTI Thermal Underwear

• Construction: Fleece-lined thermal fabric
• Stretch: Four-way stretch
• Moisture Control: Sweat-wicking
• Use: Cold-weather layering

WEERTI Thermal Underwear is designed for hikers who prioritize warmth and comfort when temperatures drop. The fleece-lined construction provides a soft layer against the skin while helping retain body heat during cold outdoor activities. Its four-way stretch fabric allows the material to move with your body, making it suitable for walking, climbing, bending and other movements encountered on the trail. The comfortable design also makes this set practical beyond hiking.

The breathable fabric is designed to wick away sweat and help control odor, which can be useful when hiking for extended periods or wearing the same base layer throughout the day. As a layering essential, it can be worn beneath hiking pants and other cold-weather clothing without restricting movement. The fleece interior makes it particularly appealing for colder conditions, while the flexible construction helps prevent uncomfortable bunching. It is also versatile enough to use around camp, at home or as comfortable sleepwear during outdoor trips.

Pros:

• Soft fleece lining provides cozy warmth
• Four-way stretch supports unrestricted movement
• Moisture-wicking and breathable construction

Cons:

• Fleece lining can feel too warm for mild weather
• Less suitable for lightweight summer hiking

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3. Smartwool Classic

• Fabric: Merino wool
• Moisture Wicking: Yes
• Weight: Midweight
• Inseam: 30 inches

The Smartwool Classic Thermal Merino base layer bottoms are a strong option for hikers who want the natural performance of merino wool during cold-weather adventures. The merino fabric provides dependable warmth while remaining comfortable against the skin, making these bottoms useful for long days on the trail. Their midweight construction offers a practical balance between insulation and versatility, especially when worn underneath hiking pants in cool or cold conditions.

Moisture-wicking performance helps manage perspiration during active hiking, while the merino construction is well suited to changing conditions on the trail. The improved fit is designed to provide comfortable movement as you hike, climb and navigate uneven terrain. A fly opening adds everyday convenience, and the 30-inch inseam provides coverage for effective layering. Whether you’re heading out for a winter hike, a cool-weather backpacking trip or an active day outdoors, these bottoms provide a reliable foundation layer.

Pros:

• Merino wool provides natural warmth
• Moisture-wicking performance for active hiking
• Midweight design works well in cold conditions

Cons:

• May require more care than synthetic fabrics
• Midweight construction can be warm for mild weather

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4. REI Co-op Lightweight

• Fabric: 94% recycled polyester/6% spandex
• UPF Rating: 50+
• Quick Drying: Yes
• Weight: Lightweight

The REI Co-op Lightweight base layer bottoms are designed for hikers who need comfortable coverage without the bulk of a heavier thermal layer. The recycled polyester and spandex fabric provides a flexible fit that moves easily with your body, making these bottoms suitable for trekking, climbing and active days outdoors. Their lightweight construction is particularly useful for moderate temperatures when you want a base layer that adds comfort without excessive insulation.

Moisture-wicking and quick-drying properties help manage sweat during demanding hikes, while the UPF 50+ rating provides added sun protection when trails are exposed to strong sunlight. These features make the bottoms versatile enough for mountain routes, open desert trails and other environments where temperatures can change throughout the day. The fly opening adds convenience, while the 28- and 31-inch inseam options provide useful fit choices. They are a practical option for hikers who prefer lightweight synthetic performance and easy layering.

Pros:

• Lightweight for moderate-weather hiking
• Quick-drying and moisture-wicking fabric
• UPF 50+ sun protection

Cons:

• Less insulating than midweight thermal bottoms
• May not provide enough warmth for severe cold

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5. Smartwool Classic All-Season

• Fabric: 88% merino wool/12% nylon
• Moisture Wicking: Yes
• Weight: Lightweight
• Inseam: 30.75 inches

The Smartwool Classic All-Season Merino Base Layer bottoms are a versatile choice for hikers who need a lightweight layer across changing seasons. Made from a blend of 88% merino wool and 12% nylon, these bottoms combine the natural comfort and moisture management of merino with added durability from nylon. The lightweight construction makes them suitable for cool-weather hiking without creating excessive bulk underneath hiking pants.

The four-season design makes these bottoms useful for more than winter adventures. They can provide comfortable coverage on snowy trails while also working well on cooler summer nights around camp. Moisture-wicking performance helps manage perspiration during active hiking, while the updated fit supports comfortable movement throughout the day. The 30.75-inch inseam provides useful coverage for layering, and the fly opening adds convenience. For hikers who want one base layer that can handle a broad range of outdoor conditions, this merino-nylon blend offers a practical combination of comfort, durability and versatility.

Pros:

• Versatile lightweight design for four seasons
• Merino wool provides natural moisture management
• Nylon blend adds durability

Cons:

• Not as warm as heavier thermal layers
• May be less suitable for extremely cold conditions

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6. Icebreaker Merino 200 Oasis

• Fabric: Merino jersey
• Weight: 200-weight
• Style: Base layer leggings
• Fly: Yes

The Icebreaker Merino 200 Oasis leggings are a dependable choice for hikers who need a comfortable foundation layer in cold weather. Made with merino jersey fabric, these leggings provide natural warmth while maintaining a soft feel against the skin. The 200-weight construction offers a useful balance between insulation and everyday versatility, making them suitable for layering beneath hiking pants during chilly trail days. Their updated design is focused on improving overall comfort during extended wear.

The fly-equipped design adds convenience when you’re spending long hours outdoors, while the leggings provide a close-fitting foundation beneath additional clothing. Merino is particularly useful for active outdoor layering because it can help regulate comfort as activity levels and temperatures change. These leggings work well for hiking, camping and other cold-weather adventures where dependable base-layer performance is important. For hikers who prefer natural fibers over synthetic thermal fabrics, the Merino 200 Oasis offers a comfortable and practical option for building a cold-weather layering system.

Pros:

• Soft merino fabric feels comfortable against skin
• 200-weight construction provides useful warmth
• Good foundation for cold-weather layering

Cons:

• May be too warm for mild-weather hiking
• Merino can require more careful washing than synthetics

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7. Arc’teryx Rho LT

• Fabric: 84% polyester/16% elastane
• Weight: Lightweight
• Moisture Wicking: Yes
• Quick Drying: Yes

The Arc’teryx Rho LT bottoms are designed for hikers who want lightweight thermal protection during low- and medium-output activities. Made from Torrent 190 fabric, the polyester and elastane blend provides a soft feel while allowing the bottoms to move comfortably with the body. Their lightweight construction adds warmth without the bulk associated with heavier thermal underwear, making them a practical choice beneath hiking pants when temperatures are cold but activity levels remain moderate.

Moisture-wicking performance helps move perspiration away from the skin, while quick-drying properties are useful when hiking conditions become more demanding. The elastane content also contributes to flexible movement when walking, climbing or navigating uneven terrain. One limitation is the absence of a fly opening, which may be less convenient for some users. Overall, these bottoms are a strong option for hikers looking for a streamlined synthetic base layer that combines warmth, moisture management and mobility without adding unnecessary weight to their layering system.

Pros:

• Lightweight warmth without excessive bulk
• Quick-drying and moisture-wicking fabric
• Stretchy construction supports easy movement

Cons:

• No fly opening
• May provide less insulation than heavier base layers

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8. SIMIYA Thermal Underwear Set

• Construction: Fleece-lined thermal set
• Stretch: Four-way stretch
• Fit: Lightweight and snug
• Care: Machine washable

The SIMIYA Thermal Underwear Set is designed for women who want a warm, comfortable base layer for cold-weather hiking and outdoor activities. Its soft fleece inner layer is designed to retain body heat while providing a comfortable feel against the skin. The lightweight, snug fit makes it easy to wear underneath jeans, ski pants or hiking clothing without creating excessive bulk. This close-fitting design is particularly useful when building multiple layers for cold conditions.

Four-way stretch fabric allows natural movement in different directions, helping reduce bunching during hiking, skiing, running and camping. The fabric is also designed to wick moisture away from the skin, helping maintain a drier and more comfortable feel during active use. A tagless design helps reduce potential irritation, while the machine-washable construction makes routine care convenient. For hikers looking for an affordable fleece-lined set that combines warmth, flexibility and easy layering, the SIMIYA set is a versatile cold-weather option.

Pros:

• Soft fleece lining provides extra warmth
• Four-way stretch supports unrestricted movement
• Lightweight design layers easily under clothing

Cons:

• Fleece may be too warm during high-output activities
• Less suited to mild-weather hiking

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9. GAYHAY Thermal Underwear Set

• Fleece Weight: 210 GSM
• Stretch: Four-way stretch
• Design: Flat-seam construction
• Set: Long-sleeve top and leggings

The GAYHAY Thermal Underwear Set is built for women who need cozy insulation without the bulk of heavy winter clothing. Its 210 GSM plush fleece-lined interior provides a warm, soft layer against the skin, making it well suited to cold-weather hiking and other outdoor activities. The lightweight insulation can be worn underneath sweaters, coats and ski clothing, providing an additional layer of warmth without significantly restricting movement.

Four-way stretch fabric allows the top and leggings to move naturally with the body, which is useful when hiking, exercising or handling outdoor gear. The soft waistband is designed to stay in place without digging into the skin, while flat seams help reduce friction during extended wear. The two-piece design can also serve as comfortable sleepwear or loungewear during camping trips. For hikers who prioritize fleece comfort and flexible movement in cold conditions, this thermal set offers a practical combination of warmth, comfort and layering versatility.

Pros:

• Warm 210 GSM fleece-lined interior
• Four-way stretch provides comfortable mobility
• Flat seams help reduce friction

Cons:

• May be too warm for intense hiking
• Fleece construction adds more warmth than lightweight base layers

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10. CL convallaria Long Johns Set

• Fabric: 90% polyester/10% spandex
• Construction: Fleece-lined
• Stretch: Four-way stretch
• Care: Machine washable

The CL convallaria Long Johns Set is designed to provide substantial warmth and comfort for cold-weather hiking and winter activities. Made from a soft polyester and spandex blend with a cozy fleece interior, the set is designed to retain body heat while remaining flexible enough for active use. The four-way stretch construction allows comfortable movement when reaching, squatting and walking, making it suitable for outdoor adventures where unrestricted mobility is important.

Moisture-wicking fibers help move sweat away from the skin, reducing the sticky or clammy feeling that can develop during prolonged activity. The fitted design works well beneath snow pants, hiking pants and other winter clothing, while the elastic waistband helps keep the bottoms securely in place. High-quality stitching is designed to handle active use, and the tagless construction helps improve comfort during extended wear. For hikers looking for a warm, flexible and easy-care thermal set for cold conditions, the CL convallaria Long Johns offer practical everyday performance.

Pros:

• Cozy fleece adds warmth for cold conditions
• Four-way stretch supports active movement
• Moisture-wicking fabric helps keep skin dry

Cons:

• May be too warm for moderate temperatures
• Polyester construction may feel less natural than merino

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How to Choose Thermal Underwear for Hiking

When you’re out on the trail, the last thing you want is to feel cold, clammy, or restricted. Thermal underwear—also called a base layer—is the foundation of any effective hiking clothing system. It sits directly against your skin, and its job is to manage moisture, regulate temperature, and keep you comfortable through changing conditions. But with so many fabrics, weights, and features on the market, choosing the right pair can feel overwhelming. This guide will walk you through everything you need to know to make an informed decision.

Why Thermal Underwear Matters for Hiking

Hiking is a stop-and-go activity. You generate a lot of heat while climbing, then cool down quickly when you pause to rest or take in the view. Without a proper base layer, sweat stays trapped against your skin. Once you stop moving, that moisture chills rapidly, leading to that dreaded post-hike shiver. A good thermal layer wicks sweat away, dries quickly, and provides just enough insulation without overheating. It’s not just about warmth—it’s about moisture management and temperature regulation.

Key Factors to Consider

Before you buy, evaluate these core characteristics. They determine how the garment performs on the trail.

1. Fabric Material

The fabric is the single most important factor. There are three main categories:

• Merino Wool: Naturally odor-resistant, breathable, and soft. It regulates temperature well across a wide range of conditions. It’s more expensive and less durable than synthetics, but many hikers swear by it for multi-day trips.
• Synthetic Fabrics (Polyester, Nylon, Spandex Blends): Excellent moisture-wicking, quick-drying, and durable. They are affordable and hold their shape well. However, they tend to retain odor over time, so they need frequent washing.
• Silk: Lightweight and luxurious, silk is a good insulator for its weight, but it’s not as durable and doesn’t wick as effectively as merino or synthetics. It’s best for mild conditions or as a liner.

Many high-quality base layers blend materials—like merino wool with a small percentage of synthetic for durability—to get the best of both worlds.

2. Weight and Warmth

Thermal underwear comes in different weights, typically measured in grams per square meter (GSM). Choosing the right weight depends on the season and your activity level.

Weight Category	Typical GSM	Best Use	Pros	Cons
Lightweight	150–200	Summer hikes, high-output activities, mild climates	Breathable, quick-drying, minimal bulk	Little insulation when stationary
Midweight	200–300	Spring, fall, cool summer mornings, moderate activity	Versatile, good balance of warmth and breathability	Can be too warm for intense climbing in summer
Heavyweight	300+	Winter hiking, cold-weather camping, low activity	Excellent insulation, cozy	Bulky, less breathable, slower to dry

For most three-season hiking, a midweight base layer is the sweet spot. If you run hot, go lightweight. If you’re heading into freezing temperatures, consider heavyweight or layering a lightweight under a midweight.

3. Fit and Mobility

Your thermal underwear should fit snugly but not restrictively. It needs to be in close contact with your skin to wick moisture effectively, but if it’s too tight, it can limit circulation and range of motion. Look for:

• Flatlock seams: These lie flat against the skin to prevent chafing, especially under a backpack.
• Gusseted crotch: Provides freedom of movement for high steps and scrambles.
• Thumb loops: Help keep sleeves in place and add warmth for your hands.
• Articulated knees and elbows: Allow the fabric to bend with your body without pulling.

Try on the base layer and move around—squat, reach, twist. If it feels restrictive or bunches up, size up or choose a different cut.

4. Moisture Management and Breathability

Moisture management is non-negotiable. The fabric should pull sweat away from your skin and spread it across the surface to evaporate quickly. Look for terms like “moisture-wicking,” “quick-drying,” and “breathable.” Merino wool excels here naturally, while synthetics are engineered with capillary action in mind. Avoid cotton at all costs—it absorbs sweat, loses insulation when wet, and dries slowly, making it dangerous in cold conditions.

5. Odor Control

On multi-day hikes, odor can become a real issue. Merino wool naturally resists odor-causing bacteria, so it can be worn for several days without washing. Synthetics, on the other hand, tend to develop a persistent funk. Some synthetic fabrics are treated with antimicrobial finishes (like silver or polygiene), but these treatments can wash out over time. If you prioritize freshness, merino is the better choice.

6. Durability

Hiking gear takes a beating. Look for reinforced areas in high-wear zones like under the arms and between the thighs. Merino wool is softer but can develop holes with heavy use; synthetic blends with a bit of spandex or nylon tend to be more abrasion-resistant. Check reviews for long-term durability before committing.

7. Care and Maintenance

How you care for your thermal underwear affects its lifespan. Always follow the manufacturer’s instructions, but here are general guidelines:

• Merino wool: Machine wash cold on gentle cycle, use a wool-specific detergent, and lay flat to dry. Avoid fabric softeners and bleach.
• Synthetics: Machine wash warm or cold, tumble dry low. Avoid high heat, which can damage the fabric’s elasticity.
• Silk: Hand wash or use a delicate cycle, and hang dry away from direct sunlight.

For multi-day trips, consider bringing a spare pair or rinsing your base layer in a stream and letting it dry overnight. Quick-drying fabrics make this feasible.

Men’s vs. Women’s Thermal Underwear

While the principles are the same, fit differs. Women’s base layers are typically cut with narrower shoulders, a shorter torso, and more room in the hips and chest. Men’s versions have broader shoulders and a straighter cut. Some brands offer unisex options, but if you want optimal fit and comfort, choose a gender-specific design.

Layering System: Where Thermal Underwear Fits

Thermal underwear is your base layer. Over it, you’ll wear a mid layer (like a fleece or puffy jacket) for insulation, and an outer shell (like a rain jacket) for protection from wind and precipitation. Your base layer should be thin enough to fit comfortably under these without bunching. Avoid bulky seams or excess fabric that can create pressure points under a pack.

Common Mistakes to Avoid

• Wearing cotton: As mentioned, cotton is a poor choice for hiking. It holds moisture and loses insulation, which can lead to hypothermia.
• Choosing a layer that’s too tight: Restricts movement and can cause chafing.
• Choosing a layer that’s too loose: Won’t wick effectively and can bunch under other layers.
• Ignoring the weather forecast: A heavyweight base layer in summer will cause overheating. Match the weight to conditions.
• Not testing before a big trip: Always do a short hike or wear your base layer around the house to ensure it’s comfortable before relying on it in the backcountry.

Top Features to Look For

When scanning product descriptions, prioritize these features:

• Flatlock seams to prevent chafing.
• Tagless labels for added comfort.
• Thumb loops for hand warmth and sleeve security.
• Gusseted crotch for mobility.
• Antimicrobial treatment (for synthetics) or natural odor resistance (merino).
• UPF rating if you’ll be hiking in strong sun with the base layer exposed.

Budget Considerations

You can spend anywhere from $30 to over $150 on a base layer. Higher prices often mean better materials (like premium merino), superior construction, and enhanced durability. However, affordable synthetics from reputable brands can perform very well for casual hikers. If you hike frequently or in extreme conditions, investing in a high-quality pair is worth it. For occasional trips, a mid-range synthetic or a merino blend offers good value.

How to Test Thermal Underwear Before You Buy

If shopping online, read reviews carefully—especially those from hikers who describe their experience in real conditions. Look for comments on fit, warmth, moisture management, and durability. If possible, visit a store and try on different brands. Pay attention to how the fabric feels against your skin and whether the seams irritate. Some brands have a reputation for running small or large, so check sizing charts and return policies.

Final Checklist for Choosing Thermal Underwear

• Material: Merino wool, synthetic, or blend (avoid cotton).
• Weight: Lightweight, midweight, or heavyweight based on season and activity.
• Fit: Snug but not restrictive; flatlock seams; gusseted crotch.
• Moisture management: Wicks sweat and dries quickly.
• Odor control: Natural (merino) or treated (synthetic).
• Durability: Reinforced high-wear areas.
• Care: Easy to wash and dry on the trail or at home.
• Layering compatibility: Fits smoothly under mid and outer layers.

Choosing the right thermal underwear for hiking doesn’t have to be complicated. Focus on the fabric, weight, and fit, and consider the conditions you’ll be hiking in. A good base layer will keep you dry, warm, and comfortable, allowing you to enjoy the trail without distraction. Whether you opt for the natural performance of merino wool or the rugged reliability of synthetics, investing time in this foundational piece of gear will pay off on every adventure.