Winter Swimming is the activity of swimming during the winter season, typically in outdoor locations (Open water swimming), or in unheated pools or lidos. Many winter swimmers swim with standard swimming costumes rather than with wetsuits or other thermal protection.

In colder countries winter swimming may be synonymous with Ice swimming. Ice swimming is swimming in a body of water with a frozen crust of ice, which requires either breaking the ice or entering where a spring keeps the water from freezing over. This may also be simulated by a pool of water at 0 °C, the temperature at which water freezes. Ice swimming is possible because the freezing of water releases heat, so that the water does not refreeze instantly. Ice swimmers typically dip their heads below the surface to ensure that they are completely wet, whether or not the water is over their heads

A polar bear plunge is an event held during the winter where participants do a brief dip in cold water. Polar bear plunges are usually held to raise money for a charitable organization.