A union suit is a type of one-piece long underwear long favored by men in North America until recent times. Traditionally made of red flannel with long arms and long legs, it also traditionally buttoned up the front and had a button-up rear access hatch for defecation.

It was not uncommon up until the mid-1900s for rural men to wear the same union suit continuously all week, or even all winter.

Normally, no other type of underwear was worn with it. One of the major events of the spring was the time when the union suits were removed, washed, and put away for the summer.

Today, most people -- both men and women --favor two-piece long underwear, also known as "long johns".