The 6 Best Thermal Underwear of 2025
We sent 14 testers out in the winter to go hiking, running, cross-country skiing, and downhill skiing, as well as to shovel snow. And we recommend the Ridge Merino Aspect Midweight Merino Wool Base Layer Long Sleeve Shirt, for men and women. The Aspect shirt offers the most versatile styling for casual and recreational use, and it’s warm yet breathable.
What about base-layer bottoms? We discovered that most people engaged in such vigorous outdoor activities will overheat when wearing base-layer bottoms unless temps are well below freezing. Most of our base-layer shirt picks have matching pants (though we didn’t test them); if you really need a pair, try those.
In this guide’s title, we refer to these layers as “thermal underwear” because that’s what most people search for when they’re looking for something to wear while they’re active. And that’s what base layers are: clothing that helps to regulate body heat by managing moisture while you’re active.
If you’re looking for classic cotton long johns with a matching top—better suited to lounging around the house than running outdoors—you’ll find those in our guide to pajamas.
Top pick
This breathable, durable merino-blend shirt, which comes in basic colors, can be worn alone or layered. It’s available in a wide range of sizes, but we’d still like to see some larger sizes.
This is the same shirt as the men’s version, with a more-tapered cut.
The Ridge Merino Aspect Midweight Wool Base Layer Long Sleeve Shirt, for men and women, is our most versatile pick, due to its breathable, merino-based blend and understated look. This is a city-to-trail shirt because it can navigate cold-weather running, skiing, backpacking, and work (plus dinner out afterward) equally well. Also, it’s reasonably priced.
This shirt’s fit is on the slim side. But, unlike some of the competition, the Aspect shirt is not so tight that it can be pigeonholed solely as an outdoor performance piece. It comes in enough basic colors to work with most of your wardrobe.
The merino wool manages cold-weather sweating, and some nylon helps with durability and shape retention after stretching. Ridge offers the crewneck version of this shirt in a men’s and women’s quarter zip. It’s also available in a version with a neck warmer (women’s) and a full hood (men’s).
Although this shirt comes in a wider range of sizes than many competitors, there is still room for larger offerings. If you end up with the wrong size and need to return it, Ridge Merino is the only company among our picks that offers free returns, including a return label, to ensure that you get the right fit.
The Aspect shirt is machine-washable, but since it’s mostly merino wool, the company recommends air-drying it to preserve the merino fibers and extend the garment’s lifespan.
Budget pick
This synthetic option is for those who prefer to avoid merino wool. It’s also for people who want an easy-care base layer. It looks more like “underwear” than our top pick, but its fit is similarly slim, and it can be layered for more warmth.
Like our top pick, the women’s version is the same shirt as the men’s version, in a more-tapered cut.
The REI Co-op Midweight Base Long-Sleeve Base Layer Top, for men and women, is the best synthetic alternative to our top pick. It costs less, too, and for those who want a softer fabric—at the expense of some odor control and slightly lower-performing moisture management—it could be a good choice. It looks more like underwear than our top pick, though, so it may not be as versatile.
The REI Co-op Midweight shirt is made from synthetic fibers, which typically dry faster than merino because they don’t soak up water as much as hollow merino fibers. However, the midweight fabric is a little heavier than that of our top picks, and the drying time was only slightly better when we were actively sweating in it. Though some synthetic fabrics can be more breathable than merino—the knit pattern and tightness largely determine how well a base layer dumps excess heat—the REI Co-op Midweight shirt breathed a little less effectively than our top pick.
Since this shirt lacks the natural odor control of merino wool, it may need to be washed more often than our top pick. But the REI Co-op Midweight can go in a home dryer, so it’s easier to launder.
REI’s returns window is 365 days for members and 90 days for nonmembers; this is the longest window we found. However, if you don’t live close enough to an REI store to return an item in person, note that REI charges a fee for a mailing label.
The REI Co-op Midweight shirt comes in a wider size range, for both men and women, than our top pick; there are also some tall options for men.
Best for…
This soft, 100% merino wool top is thicker than our other picks. So it’s a great shirt for those looking to get more warmth from their base layer—and for those who care less about performance during activities that involve sweating.
Though it’s differently named, this shirt is the same as the men’s version, with a more-tapered fit.
The Minus33 Midweight Merino Wool shirt, for men and women, is actually a heavier option than either the Ridge Merino Aspect or the REI Co-op Midweight base layers. The Minus33 Midweight shirt’s fabric works better in less-sweaty situations, such as everyday, around-town use; fairly stationary styles of hunting or fishing; or resort skiing and snowboarding on very cold days (when you’re wearing all of your layers to stay warm on the lifts). The Minus33 shirt is a little more expensive than our top pick, but you end up with a warmer shirt.
This base layer is also available in a quarter-zip style for men and women.
The Minus33 base layer is the only 100% merino wool option among our picks. This shirt’s fabric isn’t as durable as shirts made with a merino-and-nylon blend (like the Ridge Merino Aspect) or a synthetic (such as the REI Co-op Midweight). And the Minus33 shirt’s fabric doesn’t retain its shape as well. But the thicker fabric helps mitigate those drawbacks. Also, air-drying will prolong this shirt’s life, as it does for the Ridge Merino Aspect shirt.
The Minus33 shirt fits the most loosely of our picks, too. So it’s not as good at moisture management as more form-fitting options, like our picks from REI Co-op and (even more so) Ridge Merino. However, due to its looser fit, this shirt has the most casual fit for daily wear.
Among our picks, the Minus33 shirt offers the largest overall size—up to a 61-inch chest (size 6X) for the men’s version. Unfortunately, this isn’t true of the women’s version, which goes up to a 47-inch chest (size 3X) only.
Of our picks, Minus33 has the shortest return window—30 days—and it offers no free returns. The shirts are carried by a lot of retailers, though, so you may benefit from a better policy by shopping around online or visiting a brick-and-mortar store.
