A swimsuit is an item of clothing designed to be worn by people engaging in a water-based activity or water sports, such as swimming, diving and surfing, or sun-orientated activities, such as sun bathing. Different types may be worn by men, women, and children. A swimsuit can be described by various names, some of which are used only in particular locations, including swimwear, bathing suit, swimming costume, bathing costume, swimming suit, swimmers, swimming togs, bathers, cossie (short for "costume"), or swimming trunks for men, besides others.

A swimsuit can be worn as an undergarment in sports that sometimes require a wetsuit or drysuit such as cold water swimming, water skiing, scuba diving, surfing, and wakeboarding. Swimsuits may also be worn to display the wearer's physical attributes, as in the case of beauty pageants or bodybuilding contests, and glamour photography and magazines like the annual Sports Illustrated Swimsuit Issue featuring models and sports personalities in swimsuits.

There is a very wide range of styles of modern swimsuits available, which vary as to body coverage and materials. The choice of style may depend on community standards of modesty, as well as current fashions, and personal preferences. The choice will also consider the occasion, for example whether it is to be worn for a passive occasion such as sunbathing or for an activity such as surfing or swimsuit competition. Swimwear universally covers at least the wearer's crotch area. For males, swimwear today usually leaves the upper body uncovered, while swimsuits for females usually cover at least the nipples, except in the case of a topless swimsuit.