Long underwear, also called long johns or thermal underwear, is a style of two-piece underwear with long legs and long sleeves that is normally worn during cold weather. It is commonly worn by people under their clothes in cold countries.

In the United States, it is usually made from a cotton or cotton-polyester-blend fabric with a waffle weave texture, although some varieties are also made from flannel, particularly the union suit, while many newer varieties are made from polyester, such as the Capilene trade name.

European manufacturers use wool blends or even 100% wool, usually Merino or other high-quality wool. Some models might include a thin layer of polyester to transport moisture away from the skin. Wool, in addition to being fire retardant, provides highly effective insulation and will keep its insulating properties even when wet, as opposed to cotton.

The type known as "thermal underwear" is made from two-ply fabric of either a wool layer and an artificial fibre, only wool or – again mostly in the U.S. – two layers of only artificial fibres, which uses trapped body heat to insulate against cold air.