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 and styles may be worn by men, women, and children. Swimsuits can be described by various names, some of which are used only in particular locations or for particular types of suit, including swimwear, bathing suit, bathing attire, swimming costume, bathing costume, swimming suit, swimmers, swimming togs, bathers, cossie (short for "costume"), or swimming trunks (swimwear that resembles shorts), 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. According to international standards for social etiquette and swimming functionality, male swimwear usually leaves the upper body uncovered, while swimsuits for females usually cover the chest or at least the nipples.

Prior to the 1930s, swimsuits were typically made of wool; however, such suits did not hug the body and became heavy with water.^{[citation needed]}

Rayon began to be used in the 1920s in the manufacture of tight-fitting swimsuits, but its durability, especially when wet, proved problematic, with jersey and silk also sometimes being used.

In the 1930s, new materials were being developed and used in swimwear, particularly latex and nylon. Swimsuits incorporating nylon dry more quickly then swimsuits made of common organic fibers, which can create a more comfortable experience for the wearer. Swimsuits gradually began hugging the body, especially women's swimsuits.

In the 1960s, spandex (Lycra) began to be used in swimsuits, usually combined with nylon, to make them fit snugly to the body. However, spandex is not particularly strong or durable, especially in chlorinated water in swimming pools and hot tubs.^{[citation needed]}

Polyester is becoming more common as a durable, lightweight fabric for swimsuits, although it is not as stretchy as spandex.^{[citation needed]}