Our editors independently research, test, and recommend the best products and services; you can learn more about our review process here. We may receive commissions on purchases made from our chosen links.
Best Overall: Smartwool Merino 250 Base Layer at Amazon
"The warmest base-layer option that Smartwool offers."
Runner-Up, Best Overall: Duofold Double-Layer Thermal Pant at Amazon
"Consists of a cotton inner layer and a merino wool outer."
Best Budget: Hanes Waffle Knit Thermal Pant at Walmart
"Adapts to temperature and activity for all-day comfort."
Best for Women: L.L.Bean Heavyweight Base Layer Pants at L.L.Bean
"Extremely breathable and odor-resistant, while wicking moisture away."
Best for Men: Patagonia Capilene Midweight Zip-Neck Top at Backcountry
"Its genius structure allows for easy venting when you get sweaty."
Best Leggings: Uniqlo Heattech Stirrup Leggings at Uniqlo
"Fashion-conscious leggings with an array of cold-weather comfort features."
"Thin, yet warm and completely itch-free."
Best Onesie: Smartwool Merino 250 Hooded One-Piece at Nordstrom
"Feels like you're wrapped up in the world’s warmest blanket."
Best Wool Blend: Icebreaker Women’s BodyFitZone Leggings at Amazon
"Perfect for year-round use, thanks to its lightweight fabric that actively ventilates."
"Stylishly designed to retain body heat and efficiently wick away sweat."
Our Top Picks
Best Overall: Smartwool Merino 250 Base Layer
Hands down, the best long underwear for extreme temperatures is Smartwool’s Merino 250 Top. Not only is this midweight, merino wool base-layer the warmest base-layer option that Smartwool offers, but it’s also highly breathable and ultra-comfy, with none of the itchiness of other thermal underwear fabrics. Shoulder panels help to reduce chafing and optimize comfort, while flatlock stitching eliminates abrasion and enhances fit by reducing bulky seams. Plus, you’ll stay extra-protected from the sun’s harmful UV rays, thanks to the top’s UPF 50+ layer of fabric. All in all, when it comes to sheer quality and durability, nobody does thermal underwear better than Smartwool.
Runner-Up, Best Overall: Duofold Midweight Double-Layer Thermal Pant
Duofold’s Midweight Double-Layer Thermal Pants are some of the company's warmest styles, and they’re the perfect choice for those who are looking for a budget-minded pick for a cold vacation — or just need to stock up on a few pairs to get through winter. Its two layers consist of a 100 percent cotton inner layer for softness, along with merino wool in the outer layer for natural insulation. The anti-microbial fabric keeps smells out, while moisture-wicking tech keeps you warm — and sweat-free. The flatlock seams prevent chafing, no matter what activity you’re up to, and the tagless label doesn’t irritate or itch skin. You won’t want to wear these solo in the cold, as they’re not incredibly windproof, but under a thicker outer layer, they do great. Keep in mind that for long legs, these might run a tiny bit on the short side. A similar version for women is available here.
Best Budget: Hanes Waffle Knit Thermal Pant
Prices for thermal underwear can vary widely, but you don’t have to break the bank to stay warm and toasty (or dry and cool) — Hanes makes a great thermal pant for under $20. The Women’s Waffle Knit Thermal Pant is made from a buttery-soft organic cotton blend and X-Temp technology that’s designed to adapt to temperature and activity, keeping you cool in warm weather and dry for all-day comfort, making these pants versatile to wear year-round. Don't worry about it smelling, either: the FreshIQ advanced odor protection technology does a good job of attacking odor-causing bacteria. A men's version is available here.
Best for Women: L.L.Bean Heavyweight Base Layer Pants
L.L.Bean does base layers incredibly well, and the Heavyweight Base Layer Pants for Women are no exception. These pants are easily one of the most reliable choices for thermal underwear, especially if you’ll be in extra-frigid temperatures. Featuring a 220-gram blend of 48 percent polyester, 48 percent Repreve polyester, and 4 percent elastane, these thermal pants will keep you super-warm on the inside, regardless of how cold it gets outside. The pants wick moisture away from your body, making them extremely breathable and odor-resistant. As a bonus, L.L.Bean also uses recycled content when making these pants (each pair uses 10 recycled bottles) to minimize the impact on the environment.
See more reviews of our favorite base layers available for purchase.
Best for Men: Patagonia Capilene Midweight Zip-Neck Long Underwear Top
Patagonia is the crème de la crème of activewear, so it’s no surprise that the Capilene Midweight Zip-Neck Top for Men is a winner. To begin with, there’s the specialized, quick-drying Polartec Power Grid fabric that boasts warmth, breathability, and optimal moisture-wicking performance. There’s also the top's genius structure — a long, center-front zipper that allows for easy venting when you get a tad too sweaty, as well as full underarm gussets and raglan shoulder seams that provide shoulder mobility. Plus, the fabric’s smooth face is also ideal for layering. This Patagonia top has it all.
Want to explore other products from the brand? Check out the best Patagonia luggage items.
Best Leggings: Uniqlo Heattech Extra Warm Pile-Lined Stirrup Leggings
For extra warmth for your lower body (without adding a bulky under-layer to your outfit), Uniqlo's Heattech Pile-Lined Stirrup Leggings for Women are a classic choice. These fashion-conscious leggings (perfect for winter skirt styles) have an array of cold-weather comfort features, like insulating, moisturizing, bio-warming, moisture-wicking, and anti-odor/anti-microbial, along with brushed lining for a boost of warmth. Even more, you can pair the stirrup fit with an extra-long, chunky, cable knit sweater for the perfect running-errands-in-subzero-temps outfit.
Best Wool: L.L.Bean Cresta Wool Midweight 250 Base Layer
The Cresta Wool Midweight 250 Base Layer is thin, yet warm — in other words, it’s exactly what you want in a base layer. Made from itch-free fine merino wool fibers, this versatile top will keep you cozy in cold weather and cool in warm weather, so it’s perfect for a range of activities and temperatures. Fine-gauge knit with flatlock seams make for an incredibly smooth fit, and thumbhole cuffs keep sleeves snug at your wrist. Unlike clothing made with traditional (read: itchy) wool, this L.L.Bean base layer feels silky-soft against your skin.
Add an extra layer and shop for the best fleece jackets on the market.
Best Onesie: Smartwool Merino 250 Hooded One-Piece Base Layer
Wearing the Smartwool Merino 250 Hooded One-Piece is more akin to wrapping yourself up in the world’s warmest, comfortable blanket than wearing traditional thermal underwear — it’s just that cozy. Made from pure merino wool, this onesie provides a high level of natural insulation and strong wicking, so it does a truly excellent job of keeping you warm, wicking moisture, and reducing odors. Best of all, unlike other thermal underwear onesies, this Smartwool one-piece is made from wool in a tight, interlock knit, so it’s not bulky.
Best Wool Blend: Icebreaker Women’s BodyFitZone 200 Zone Leggings
Whether you’re hitting the ski slopes, traipsing around on snow-covered trails, or just trying to get home from work without turning into a human icicle, the Icebreaker Women’s BodyFitZone Leggings are ideal. They’re perfect for year-round use, thanks to the lightweight merino wool fabric with strategically placed mesh panels for active ventilation. Flatlock seams prevent chafing and the LYCRA content aids in stretch and mobility. And, the Icebreaker-branded brushed elastic waistband offers ultimate comfort.
Interested in reading more reviews? Take a look at our selection of the best wool socks.
Best Style: Columbia Women’s Glacial Fleece Legging
Designed to retain body heat and efficiently wick away sweat, Columbia’s Glacial Fleece Leggings are also totally stylish. Warm, soft, and odor-free, they're crafted from lightweight microfleece, and the fit is super-flattering. You can easily layer the leggings under rain or snow pants; they’re sturdy enough for running or lounging around with, but thin enough that you can layer underpants when the temperature dips. Given how trendy these leggings are, we’re betting you’ll want to wear them on their own.
Best Alternative: Terramar Women’s Thermasilk Pointelle Pant
A natural alternative to wool thermal underwear, Terramar’s Thermasilk Pointelle leggings are crafted from 100 percent filament silk. Lightweight and stretchy, these underwear bottoms come with the EC2 Qwik-Dri Electrostatic process, which has the ability to lift the moisture molecules away from your body and break them down for super-swift evaporation. No matter what moderately cold-weather activities you’re undertaking, these Terramar leggings will keep you snug and warm — conveniently, they’re machine-washable, too. Just keep in mind that silk-based thermal underwear may not be as thick or insulating as a wool blend, so these leggings probably aren’t the best choice for subzero temperatures.
Our writers spent 3 hours researching and testing the most popular thermal underwear on the market. Before making their final recommendations, they considered 20 different thermal underwear overall, screened options from 15 to 20 different brands and manufacturers, read over 30 to 40 user reviews (both positive and negative), and tested 2 of the thermal underwear themselves All of this research adds up to recommendations you can trust.