A kayak is a small, narrow watercraft which is typically propelled by means of a double-bladed paddle. The word kayak originates from the Greenlandic word qajaq (IPA: [qajɑq]).

The traditional kayak has a covered deck and one or more cockpits, each seating one paddler. The cockpit is sometimes covered by a spray deck that prevents the entry of water from waves or spray, differentiating the craft from a canoe. The spray deck makes it possible for suitably skilled kayakers to roll the kayak: that is, to capsize and right it without it filling with water or ejecting the paddler.

Some modern boats vary considerably from a traditional design but still claim the title "kayak", for instance in eliminating the cockpit by seating the paddler on top of the boat ("sit-on-top" kayaks); having inflated air chambers surrounding the boat; replacing the single hull with twin hulls; and replacing paddles with other human-powered propulsion methods, such as foot-powered rotational propellers and "flippers". Kayaks are also being sailed, as well as propelled by means of small electric motors, and even by outboard gasoline engines.

The kayak was first used by the indigenous Aleut, Inuit, Yupik and possibly Ainu hunters in subarctic regions of the world.