A wetsuit is a garment worn to provide thermal protection while wet. It is usually made of foamed neoprene, and is worn by surfers, divers, windsurfers, canoeists, and others engaged in water sports and other activities in or on the water. Its purpose is to provide thermal insulation and protection from abrasion, ultraviolet exposure, and stings from marine organisms. It also contributes extra buoyancy. The insulation properties of neoprene foam depend mainly on bubbles of gas enclosed within the material, which reduce its ability to conduct heat. The bubbles also give the wetsuit a low density, providing buoyancy in water.

Hugh Bradner, a University of California, Berkeley, physicist, invented the modern wetsuit in 1952. Wetsuits became available in the mid-1950s and evolved as the relatively fragile foamed neoprene was first backed, and later sandwiched, with thin sheets of tougher material such as nylon or later spandex (also known as lycra). Improvements in the way joints in the wetsuit were made by gluing, taping and blind-stitching, helped the suit to remain waterproof and reduce flushing, the replacement of water trapped between suit and body by cold water from the outside. Further improvements in the seals at the neck, wrists, ankles, and zippers produced a suit known as a "semi-dry".

Different types of wetsuit are made for different uses and for different temperatures. Suits range from a thin 2mm or less "shortie", covering just the torso, upper arm, and thighs, to thick 8mm semi-dry suit covering the torso, arms, and legs, usually complemented by neoprene boots, gloves and hood. The type of the suit depends upon the temperature of the water and the depth of the planned dive.

The difference between a wetsuit and a dry suit is that a wetsuit allows water to enter the suit, though good fit limits water circulation inside the suit, and between the inside and outside of the suit, while dry suits are designed to prevent water from entering, thus keeping the undergarments dry and preserving their insulating effectiveness. Wetsuits can give adequate protection in warm to moderately cold waters. Dry suits are typically more expensive and more complex to use, but can be used where protection from lower temperatures or contaminated water is needed.

The primary function of a wetsuit is thermal insulation to keep the wearer warm in conditions where they would otherwise lose body heat rapidly due to heat transfer by relatively large quantities of water. Secondary, and incidental, functions are buoyancy and protection from some environmental hazards such as abrasion, sunburn, and to a lesser extent, wind chill. Wetsuits are used for thermal insulation for activities where the user is likely to be immersed in water, or frequently doused with heavy spray, often approaching from near-horizontal directions, where normal wet-weather clothing is unlikely to keep the water out. Activities include underwater diving, sailing, sea rescue operations, surfing, river rafting, whitewater kayaking and in some circumstances, endurance swimming.

Still water (without currents or convection) conducts heat away from the body by pure thermal diffusion, approximately 20 to 25 times more efficiently than still air. Water has a thermal conductivity of 0.58 Wm⁻¹K⁻¹ while still air has a thermal conductivity of 0.024 Wm⁻¹K⁻¹, so an unprotected person can eventually succumb to hypothermia even in warmish water on a warm day. Wetsuits are made of closed-cell foam neoprene, a synthetic rubber that contains small bubbles of nitrogen gas when made for use as insulating material (neoprene is also manufactured without foaming for many other applications where insulating qualities are not important). Nitrogen, like most gases, has very low thermal conductivity compared to water or to solids, and the small and enclosed nature of the gas bubbles minimizes heat transport through the gas by convection in the same way that cloth fabrics, fur, or feathers insulate by reducing convection of enclosed air spaces. The result is that the gas-filled cavities restrict heat transfer to mostly conduction, which is partly through bubbles of entrapped gas, thereby greatly reducing heat transfer from the body (or from the layer of warmed water trapped between the body and the wetsuit) to the colder water surrounding the wetsuit.

Uncompressed foam neoprene has a typical thermal conductivity in the region of 0.054 Wm⁻¹K⁻¹, which produces about twice the heat loss of still air, or one-tenth the loss of water. However, at a depth of about 15 metres (50 ft) of water, the thickness of a typical neoprene foam will be halved and its conductivity will be increased by about 50%, allowing heat to be lost at three times the rate at the surface. The grade of foam neoprene strongly affects insulating properties at depth, and over time. Softer, lighter, and more elastic grades contain a higher percentage of gas bubbles, and are comfortable and provide effective insulation at or near the surface where they retain much of their thickness. Areas that are significantly stretched lose thickness even before they are compressed at depth, which also reduces the insulation, and long periods under pressure and repeated compression and decompression of the neoprene foam will eventually lead to loss of volume, insulation, buoyancy and flexibility. Some bubbles will also rupture under stress and lose their gas, and the foam will start to absorb more water, further reducing insulation. Wetsuits for diving should be made from less compressible neoprene to keep their insulating qualities.

A wetsuit must have a snug fit to work efficiently when immersed; too loose a fit, particularly at the openings (wrists, ankles, neck and overlaps) will allow cold water from the outside to enter when the wearer moves. Flexible seals at the suit cuffs aid in preventing heat loss in this fashion. The elasticity of the foamed neoprene and surface textiles allow enough stretch for many people to effectively wear off-the-shelf sizes, but others have to have their suits custom fitted to get a good fit that is not too tight for comfort and safety. Places where the suit bridges a hollow tend to change volume when the wearer bends that part of the body, and the change in volume of the space under the suit works as a pump to push warm water out of the suit and suck cold water in on the opposite movement.