A rash guard, or rashguard, is a lightweight athletic shirt made from a material like nylon or polyester, engineered to have excellent wicking ability. It protects the skin from a variety of sporting related hazards, from chafing to sunburn. In addition to being practical, these shirts have also become a style statement in some parts of the world, especially around surfing communities, where they may be worn as a casual shirt in addition to a piece of sporting equipment.

One of the major advantages of a rash guard is that the lightweight design allows it to be worn even in very warm weather conditions. Many people like to use garments with built-in SPF protection in the tropics to prevent sunburn and keep their skin cool while they surf, snorkle, or swim. They can also be worn for land sports like tennis, and they may also be worn under other garments.

Classically, rash guards are worn under a wetsuit to prevent chafing. Prolonged periods of time in a wet suit can cause skin problems, especially in salt water around areas where the suit is bent, like the knees and elbows, with the wrinkled parts of the suit chafing and irritating the skin, but wearing a rash guard can prevent this. It also offers extra insulation for people using a wetsuit in very cold water.