10 Best Thermal Underwear 2026 in the United States

The Merino.tech Merino Wool Base Layer Set is a strong choice for men seeking high-quality thermal underwear made from 100% superfine merino wool. Its midweight fabric (250 g/m²) strikes a nice balance between warmth and breathability, making it suitable for cooler weather or layering in colder conditions. The material is very soft and comfortable, thanks to the fine 17.5 micron wool, which helps avoid the itchiness sometimes associated with wool. The set includes both a shirt and pants, plus a bonus merino wool beanie, which adds good value and makes it a handy gift option.

One of the biggest advantages is the natural moisture-wicking and odor-resistant properties of merino wool, which help keep you dry and fresh longer than typical cotton or synthetic fabrics. It also regulates body temperature well, allowing for comfortable wear across different seasons. Merino wool requires gentle care—hand washing and laying flat to dry is recommended to maintain performance and durability, though machine washing is possible but less ideal.

The product is designed for daily wear with attention to avoiding bunching or overheating, though checking size details carefully is advisable to ensure a snug yet comfortable fit. Durability is decent for a wool garment, but as with all fine merino wool items, it may not withstand heavy abrasion or rough use as well as some synthetic alternatives. For those seeking natural, breathable, and odor-resistant thermal underwear for regular outdoor or active use, this Merino.tech set represents a solid pick, provided it is treated with proper care to maintain its quality.

The Under Armour 3-Pack Performance Tech Solid Boxer Briefs are made from a lightweight blend of 90% recycled polyester and 10% elastane, providing a soft feel with plenty of stretch for comfort and mobility. Their 9" inseam and regular fit make them suitable for everyday wear without feeling too tight or loose. These boxer briefs feature moisture-wicking fabric that pulls sweat away from your skin and dries quickly, helping you stay fresh during active or casual use.

They also have a breathable mesh fly and gusset to enhance airflow and comfort. The absence of side and back seams helps prevent irritation, making them comfortable for long wear. The boxer briefs are designed to be durable enough to hold up through regular machine washing without losing their shape or stretch. The lightweight fabric is ideal for warmer conditions or layering, though it may not provide much insulation in very cold weather if you're seeking thermal underwear.

These briefs fit true to size and are well-suited for those looking for breathable, moisture-managing underwear with a snug but comfortable fit, ideal for sports or casual everyday wear.

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.