A sea kayak or touring kayak is a kayak used for the sport of paddling on open waters of lakes, bays, and oceans. Sea kayaks are seaworthy small boats with a covered deck and the ability to incorporate a spray deck. They trade off the manoeuvrability of whitewater kayaks for higher cruising speed, cargo capacity, ease of straight-line paddling (tracking), and comfort for long journeys.

Sea kayaks are used around the world for marine (sea) journeys from a few hours to many weeks, and can accommodate one to three paddlers along with their camping gear, food, water, and other supplies. Solo sea kayaks are 3.0–5.5 m (10–18 ft) long, while tandem craft can be up to 8 m (26 ft) long; beam widths range from 53 cm (21 in) to 91 cm (36 in).

The term "sea kayaking" may have been popularised by the 1981 book Sea Kayaking by John Dowd, who said, "It wasn't called sea kayaking until my book came out; it was called kayak touring or sea canoeing or canoe touring, blue-water paddling, coastal paddling, all those things".

Contemporary sea kayaks trace their origin to the native boats of Alaska, northern Canada, and Southwest Greenland. Inuit (formerly Eskimo) hunters developed a fast seagoing craft to hunt seals and walrus. The ancient Aleut name for an Aleutian kayak is Iqyak, and earliest models were constructed from a light wooden frame (tied together with sinew or baleen) and covered with sea mammal (sea lion or seal) hides. Archaeologists have found evidence indicating that kayaks are at least 4000 years old. Wooden kayaks and fabric kayaks on wooden frames (such as the Klepper) dominated the market up until the 1950s, when fiberglass boats were introduced. Rotomolded plastic kayaks first appeared in 1984.

Modern sea kayaks come in a wide array of materials, designs, and sizes to suit a variety of intended uses. In sea kayaking, where the designs continue along primarily traditional lines, the primary distinction is between rigid kayaks and Folding kayaks. Folding kayaks are in some ways more traditional boats, being similar in design to skin-on-frame kayaks used by native people. Modern folding kayaks use ash and birch or contemporary materials such as aluminum for the frame, and replace the sealskin covering with synthetic waterproof fabrics. Unlike rigid kayaks, folding kayaks can be easily disassembled and packed for transport. Many folding kayaks include inflatable sponsons that improve the secondary stability of the vessel, helping to prevent capsize. More recently, a class of inflatable folding kayaks has emerged, combining a more limited rigid frame with a tightly inflated skin to produce greater rigidity than an inflatable boat alone.

In recent years, there has been an increase in production of sit-on-top kayaks suitable for sea use.^{[citation needed]}

Most rigid sea kayaks also derive from the external designs of native vessels, especially those from Greenland, but the strength of modern materials such as fiberglass, rotomolded plastic and carbon fiber eliminate the need for an internal frame, though significantly increasing weight. Modern skin-on-frame sea kayaks constructed with nylon skins represent an ultralight niche within the rigid sea kayak spectrum. Some recent design innovations include:

A different class of vessel emerged in the 1960s, the surf ski, a long, narrow boat with low inherent stability that is intended for use in surf and following waves.