A sailor, seaman, mariner, or seafarer is a person who works aboard a watercraft as part of its crew, and may work in any one of a number of different fields that are related to the operation and maintenance of a ship. While the term sailor has its etymological roots from sailing, that is a time when sailing ships were the main mode of transport at sea, it now refers to the personnel of all watercraft regardless of the type of vessel, boat or ship. It encompasses people who operate ships professionally, be it for a military (navy) or civilian (merchant navy) or for sports or recreation. In a navy, there may be further distinctions: sailor may refer to any member of the navy even if they are based on land, while seaman may refer to a specific enlisted rank. Additionally, fisherman are seen as a distinct type of sailor, that is those engaged in fishing.

Sailors have existed from the earliest periods in history as people used boats for purposes such as maritime transport. Professional sailors normally undertake training or other forms of education to develop their skills. Professional sailors are also governed by regulations, including the STCW Convention.

Polynesian navigators would sail to several islands east of New Guinea by 1500 BCE. Voyages would eventually be travelled across the Polynesian triangle. This extensive wayfinding was conducted through the use of specialized watercraft, observing stars, and awareness of other natural phenomena.

There was Norse colonization of the Americas. After Erik the Red was exiled in Iceland, he sailed to Greenland and formed a settlement there. His son Leif Erikson would explore the east coast of North America around 1000 CE, naming areas Helluland, Markland, and Vinland. Thorfinn Karlsefni would later settle briefly near the areas explored by Erikson and have a son there before deciding to return to Iceland.

Ferdinand Magellan led the first expedition that circumnavigated the globe in 1519–1522.

A knowledge and understanding of seamanship is a key component of being a sailor, typically seen as a requisite for safety and efficient working at sea. While the level of knowledge will vary by the ship type and sailor, areas of knowledge include operational practices such as navigation, anchoring and other nautical operations. Sailors usually have an awareness of basic meteorology including the need to monitor the weather at sea. Passage planning, maintenance of machinery and/or rigging/sails is also necessary, depending on the type of vessel.

Working at sea safely also requires adequate clothing (especially when working outside on a vessel) or PPE depending on the job.

An important skill as a sailor when working with others (a crew) is effective communication.