10 Best Thermal Underwear 2025 in the United States

The Thermajohn Long Johns thermal underwear set is designed with cold weather in mind, making it a solid option for men looking for warmth and comfort. One of its standout features is the ultra-soft fleece lining, which ensures a cozy feel against the skin, enhancing the experience during chilly days. The material, made of 92% polyester and 8% spandex, offers a good balance of heat retention and flexibility, allowing for a comfortable fit without being restrictive. With its moisture-wicking capabilities, these long johns help keep you dry by drawing sweat away from the body, which is a big plus for anyone active outdoors. The 4-way stretch feature is also essential, as it allows for unrestricted movement, making these thermals suitable for various activities, from lounging at home to outdoor adventures.

There are a few considerations to keep in mind. While the fleece lining provides excellent warmth, it may be too warm for milder conditions, potentially leading to overheating if worn during less chilly weather. Additionally, the sizing may vary, so it’s advisable to check the fit before purchasing, as some users may find it runs a bit small. Durability over time also depends on the care it receives, given that it's machine washable; however, it’s wise to follow care instructions to maintain its quality.

The Thermajohn Long Johns are well-suited for individuals seeking reliable thermal underwear that combines warmth, comfort, and flexibility. They excel in cold weather settings but may not be the best choice for every temperature range. Ideal users would be those who require layering during winter activities or just want to stay warm at home.

The Merino.tech Merino Wool Base Layer Mens Set stands out with its use of 100% superfine merino wool, known for being soft and itch-free, ensuring all-day comfort. This set includes both a top and bottom in midweight (250 g/m²), making it versatile for varying temperatures. Additionally, the package includes a bonus pair of merino wool hiking socks, adding extra value.

The natural moisture-wicking and odor-resistant properties of merino wool make this thermal underwear an excellent choice for staying dry and fresh during active use. Its breathability helps regulate body temperature, providing comfort throughout the year. Despite being machine washable, hand washing and air drying is recommended for extended longevity, which might be inconvenient for some users.

The fit is designed to prevent rubbing, overheating, or bunching, contributing to comfort. This set would be particularly beneficial for men needing reliable and comfortable base layers for outdoor activities or everyday wear in cooler climates.

The Merino.tech Merino Wool Base Layer Women Set stands out for its high-quality 100% Merino wool construction, making it a great choice for anyone seeking comfort and functionality in thermal underwear. Available in three weight options—lightweight (165 g/m²), midweight (250 g/m²), and heavyweight (320 g/m²)—it offers versatility for different weather conditions. The midweight version, which includes a top, leggings, and hiking socks, is ideal for moderate climates and active wear.

The superfine 17.5-micron wool ensures a soft, itch-free experience, catering to those with sensitive skin. Its excellent moisture-wicking and natural odor-resistant properties keep you dry and fresh throughout the day, which is perfect for extended outdoor activities or daily wear. The set is also breathable, regulating body temperature to maintain comfort in varying conditions.

However, this product requires special care; it is recommended to hand wash and lay flat to dry, which could be inconvenient for some users. The durability is generally good, but like all wool garments, it might be prone to pilling over time. The product comes from China, which might be a consideration for those preferring locally-made goods. As a bonus, the packaging is attractive, making it a suitable gift option. This set is best suited for women looking for reliable thermal underwear that can handle both everyday use and outdoor adventures while providing superior comfort and performance.