A dry suit or drysuit provides the wearer with environmental protection by way of thermal insulation and exclusion of water, and is worn by divers, boaters, water sports enthusiasts, and others who work or play in or near cold or contaminated water. A dry suit normally protects the whole body except the head, hands, and possibly the feet. In hazmat configurations, however, all of these are covered as well.

The main difference between dry suits and wetsuits is that dry suits are designed to prevent water from entering. This generally allows better insulation, making them more suitable for use in cold water. Dry suits can be uncomfortably hot in warm or hot air, and are typically more expensive and more complex to don. For divers, they add some degree of operational complexity and hazard as the suit must be inflated and deflated with changes in depth in order to minimize "squeeze" on descent or uncontrolled rapid ascent due to excessive buoyancy, which requires additional skills for safe use. Dry suits provide passive thermal protection: Undergarments are worn for thermal insulation against heat transfer to the environment and chosen to suit expected conditions. When this is insufficient, active warming or cooling may be provided by chemical or electrically powered heating accessories.^: Ch1

The essential components are the waterproof shell, the seals, and the watertight entry closure. A number of accessories are commonly fitted, particularly to dry suits used for diving, for safety, comfort and convenience of use. Gas inflation and exhaust equipment are generally used for diving applications, primarily for maintaining the thermal insulation of the undergarments, but also for buoyancy control and to prevent squeeze.