10 Best Thermal Underwears 2026 in the United States

The Under Armour Men's Performance Tech Boxerjock Briefs are a solid choice if you're looking for lightweight and comfortable thermal underwear. Made from a blend of 90% polyester and 10% elastane, these boxer briefs offer a soft, breathable fabric that helps keep moisture away from your skin, which is great for staying dry during active use or cold weather. They feature a 6-inch inseam and a streamlined fit that's described as neither too tight nor too loose, making them comfortable to wear all day without feeling restrictive.

The fabric is stretchy in all directions, allowing for good freedom of movement, and the tagless design plus flat, chafe-free seams add to the comfort by preventing irritation. At around 9 ounces for the pack, they are lightweight enough to be worn under other layers without bulk. The waistband is designed to stay in place without rolling, which can be a common issue with some underwear.

The material is mostly synthetic polyester, which does a good job with moisture-wicking but might not feel as natural as cotton blends for some users. While these briefs are excellent for activewear and sports use, if you're seeking heavy thermal insulation for extremely cold conditions, these might be more suited as a base layer rather than a standalone warm layer. They offer a dependable balance of comfort, fit, and moisture management for everyday wear and physical activity.

The Merino.tech Merino Wool Base Layer Set is a solid choice for men needing reliable thermal underwear, especially in cold conditions. Made from 100% superfine 17.5 micron merino wool, it promises softness without the itchiness common in wool products. This midweight set, including both a shirt and pants at 250 g/m², provides good insulation while maintaining breathability, making it well-suited for active use or daily wear.

One standout feature is its natural moisture-wicking ability, which helps keep you dry by pulling sweat away from the skin, combined with odor-resistant properties that keep the fabric fresh even after prolonged use. The included merino wool beanie adds value, making the set a convenient package for cold weather. Fit appears comfortable with no bunching or overheating, which is important for layering or extended wear.

The care instructions recommend hand washing and laying flat to dry, which may be less convenient than machine wash and dry. Also, while the heavyweight option is available, this set is midweight, so it might not be warm enough for extremely cold conditions without additional layers. This set serves as a great pick for anyone looking for soft, breathable, and naturally moisture-managing thermal underwear, especially if you prefer natural fibers over synthetics.

The Merino.tech Merino Wool Base Layer Women Set is made from 100% superfine merino wool, known for its softness and comfort, which means it won’t cause itching like some other wool products. This particular set is midweight, weighing 250 g/m², making it suitable for year-round wear, especially in cooler conditions. It includes both a top and leggings, plus a pair of merino wool hiking socks, adding extra value. The fit is regular and true to size, designed for daily comfort without tightness or bunching.

A standout feature is its natural moisture-wicking ability, which helps keep sweat away from the skin, paired with odor resistance so you stay fresh during active use. The fabric is breathable and regulates body temperature well, which is great if you are hiking, skiing, or just spending time outdoors in varying weather. However, the fabric does not stretch, so it may feel a bit restrictive if you prefer more flexible clothing.

While the set can be machine washed on a delicate cycle, hand washing is recommended to maintain its quality longer. This set is an excellent choice for women looking for durable, comfortable thermal underwear that performs well in both cold and moderate temperatures, especially if they value natural fibers and odor control.