A one-piece swimsuit most commonly refers to swimwear worn primarily by women and girls when swimming in the sea or in a swimming pool, playing water polo, or for any activity in the sun, such as sun bathing. Today, the one-piece swimsuit is usually a skin-tight garment that covers the torso, although some designs expose the back or upper chest.

Before the popularity of the two-piece swimsuit, and then the bikini, virtually all women's swimwear completely covered at least the wearer's torso, and men also wore similar swimsuits. While the bikini has increasingly found popular acceptance since the 1960s, the one-piece swimsuit has maintained a place on beaches to this day.

The most common type of one-piece suit is the maillot (a term that is not generally used anymore) or tank suit, which resembles a sleeveless leotard or bodysuit. There are variants of the one-piece swimsuit, including halterneck styles and plunge front swimsuits, as well as wrap-round ("surplice") and bandeau styles. The pretzel suit is another style of the one-piece swimsuit. Recently, athletic swimsuits have used a variety of new shoulder strap styles, including the racerback, fastback, and flyback styles, some of which have also been used on other athletic wear. Another recent innovation in one-piece swimsuits is the bodyskin, which superficially resembles a unitard or wetsuit, covering the entire torso, arms, and legs. This serves to reduce friction through the water for professional swimmers. The surface is made of textured technical fabrics which are engineered to cut through the water in the same way as fish or sharkskin.