10 Best Thermal Underwear 2026 in the United States

The Merino.tech Merino Wool Base Layer Mens Set is a solid choice if you want reliable and comfortable thermal underwear made from 100% superfine merino wool. It comes in a midweight option (250 g/m²), which strikes a good balance for staying warm without feeling bulky—ideal for cooler weather or active outdoor use. The wool is soft and itch-free, making it comfortable for all-day wear, which is a big plus since some wool products can feel scratchy. Thanks to natural moisture-wicking, it helps keep you dry by pulling sweat away from your skin, and its odor-resistant qualities mean you can wear it longer without smelling, which is especially handy for hiking or multi-day activities.

The fit is designed to avoid bunching or overheating, adding to comfort during movement. It also comes with a free pair of merino wool hiking socks, which adds good value. One downside is the care instructions: while the label says machine wash, hand washing and laying flat to dry are recommended to maintain durability and shape over time, which might be a bit more effort than some users prefer.

While midweight is versatile, if you need extreme cold protection or very light layering, you would need to look at their heavyweight or lightweight versions separately. This set suits men looking for breathable, comfortable thermal underwear with natural performance features, especially for outdoor activities or extended wear.

The Cuddl Duds Women's Thermal Underwear set is designed to keep you warm and comfortable during cold weather. Made primarily of polyester with some spandex, it offers a soft, brushed fleece inside for warmth and a sleek outer layer that feels smooth against the skin. The heavyweight fabric and dense knit provide good insulation without feeling bulky, making it a practical base layer to wear under clothes or on its own while lounging. Thanks to the spandex stretch, the fit is flexible and moves with you, which is great for staying comfortable on the go.

It also includes thoughtful features like thumb holes on the sleeves and a tag-free label to avoid irritation. The moisture-wicking fabric works well to pull sweat away from your skin, helping you stay dry during activity, while the material blocks over 50% of UV rays, adding extra protection if you’re outdoors. Weighing about 15 ounces for the set, it’s substantial enough to provide warmth but not overly heavy.

This thermal underwear is suited for women looking for a cozy, practical base layer that balances comfort, moisture management, and stretch for everyday cold-weather use.

The Cuddl Duds Thermal Underwear set for women is designed to keep you warm in cold weather with a cozy brushed fleece lining and a soft matte exterior. Made from a blend of 93% polyester and 7% spandex, it offers a nice balance of warmth and stretch, making the fit comfortable and flexible for everyday activities. This two-piece set features moisture-wicking fabric that helps keep sweat away, which is great if you tend to get warm during physical activities or layering. The fabric is also tightly knit, providing protection from UV rays, a nice bonus if you spend time outdoors.

The thumbhole sleeves and modern rise leggings add to the practical and comfortable fit without adding bulk, allowing you to wear the set both as a base layer or on its own when lounging. At just under 15 ounces, it's lightweight enough for layering without feeling heavy. This thermal set is a suitable choice if you want a soft, stretchy, and warm outfit that works well for casual cold-weather wear, though for intense winter sports or heavy-duty use, you might want to look for additional durability features.