A rash guard, also known as rash vest or rashie, is an athletic shirt made of spandex and nylon or polyester. The name rash guard reflects the fact that the shirt protects the wearer against rashes caused by abrasion, or by sunburn from extended exposure to the sun, as sun protective clothing.

Rash guards (sometimes known as swim shirts) are multi-purpose shirts that are designed for all sorts of water-based activities. People use rash guards for a variety of purposes, most of which come down to preventing chafing and protecting yourself from the sun (which is why all of the rash guards listed below are designed for UV protection).

Rash guards are designed to keep you comfortable and protected from the elements during activities like surfing, swimming, and paddle boarding. Much lighter than a wetsuit (and typically sportier than a tankini), they're typically made of flexible, fast-drying material, like spandex or polyester.