Open water swimming is a swimming discipline which takes place in outdoor bodies of water such as open oceans, lakes, and rivers. Competitive open water swimming is governed by the International Swimming Federation, World Aquatics (formerly known as FINA), except when it is part of multi-sport events, which are governed by the World Triathlon. Non-competitive open water swimming may be called wild swimming.

In the first edition of the modern Olympic Games in Athens in 1896, the swimming competition was held in open water. In 2000, the Olympic Games first included a triathlon with a 1500 m swim leg, and in 2008, a 10 km open water swim. The FINA World Aquatics Championships has featured open water swimming events since 1992. The FINA World Open Water Swimming Championships was held from 2000 to 2010. Since 2007, the FINA 10 km Marathon Swimming World Cup is held in several events around the world.

Events such as the Midmar Mile in South Africa (attributed to Wayne Ridden), the Great Swim in the UK (whose idea is attributed to Colin Hill), and the Batley race have helped create and grow interest and participation in the sport.

Though most open water races do not require a specific stroke, most competitors employ the front crawl, also known as freestyle. The efficiency of this stroke was demonstrated by Gertrude Ederle, who, as the first woman to swim the English Channel, employed it to beat the existing world record by more than 2 hours.

When covering large distances, swimmers may head off course due to current, waves, wind, and poor visibility. Typically, buoys are stationed periodically across a large expanse to provide guidance. However, buoys are often invisible due to interference from choppy water and reduced visibility through goggles. Swimmers are encouraged to 'triangulate' by looking for two aligned, easily visible objects on land that are directly behind the destination (such as the end of a pier as it lines up with a hilltop), and to make sure they continue to appear aligned during the race.

Drafting, which is prohibited by some race regulations, is the technique of following another swimmer so closely that water resistance is reduced. When swimming closely alongside or behind a swimmer in the lead swimmer's wake, resistance is reduced and the amount of effort to swim at the same speed is correspondingly reduced. In calmer conditions, or when facing surface chop, swimmers can also significantly benefit from swimming immediately behind or closely alongside a swimmer of comparable or faster speed. Not all race organizers permit drafting, and swimmers can run the risk of disqualification if they are caught.

When entering the water, swimmers can use techniques to take advantage of the shallow water. One such technique is walking along the bottom. Another technique is "dolphining", which involves diving down to the bottom and launching oneself upwards and forwards. This technique can also help to avoid incoming waves. When exiting the water, swimmers can body surf to take advantage of waves.

The equipment allowed in a race depends on the sanctioning body and/or the race organizers. For example, races may have divisions for wetsuits and/or relays; may require escort boats / kayakers / paddleboards; and may require specifically colored swim caps. Some swimmers tend to keep it simple, using a basic swimsuit, goggles, and swim cap. Many records are based on that attire, which is known as 'channel attire' because it is stipulated in the rules for English Channel crossings and the rules for other long swims. Swimmers often utilize changing robes to dry off and change into or out of their swimwear attire in often cold and public places.