Outrigger boats are various watercraft featuring one or more lateral support floats known as outriggers, which are fastened to one or both sides of the main hull. They can range from small dugout canoes to large plank-built vessels. Outrigger boats can also vary in their configuration, from the ancestral double-hull configuration (catamarans), to single-outrigger vessels prevalent in the Pacific Islands and Madagascar, to the double-outrigger vessels (trimarans) prevalent in Island Southeast Asia. They are traditionally fitted with Austronesian sails, like the crab claw sails and tanja sails, but in modern times are often fitted with petrol engines.

Unlike a single-hulled vessel, an outrigger or double-hull vessel generates stability as a result of the distance between its hulls rather than due to the shape of each individual hull. As such, the hulls of outrigger or double-hull boats are typically longer, narrower and more hydrodynamically efficient than those of single-hull vessels. Compared to other types of canoes, smaller outrigger canoes can be quite fast, yet are also capable of being paddled and sailed in rougher water. This paddling technique, however, differs greatly from kayaking or rowing. The paddle, or blade, used by the paddler is single sided, with either a straight or a double-bend shaft.

These vessels were the first true ocean-going ships, and are an important part of the Austronesian heritage. They were the vessels that enabled the Austronesian expansion from Taiwan into the islands of both the Indian and Pacific Ocean from around 3000 BC. They comprise the bulk of traditional boats in Island Southeast Asia, Island Melanesia, Micronesia, Polynesia, and Madagascar. They have spread to other cultures Austronesians came into contact with, notably in Sri Lanka and southern India as well as in the coast of East Africa. In modern times, outrigger vessels are used in the sport of sailing. Catamaran and trimaran configurations are also widely used for high speed craft.

Outrigger boats were originally developed by the Austronesian-speaking peoples of the islands of Southeast Asia for sea travel. It is believed that the use of outriggers may have been initially caused by the need for stability on small watercraft after the invention of crab claw sails some time around 1500 BCE.

Outrigger boats were essential in the transportation of Austronesians both eastward to Polynesia and New Zealand and westward across the Indian Ocean as far as Madagascar during the Austronesian migration period. The Austronesian peoples (Malagasy, Maritime Southeast Asian, Micronesian, Melanesian, Taiwanese indigenous peoples, and Polynesian peoples) continue to be the primary users of the outrigger boats.

The simplest form of all ancestral Austronesian boats had five parts. The bottom part consists of single piece of hollowed-out log. At the sides were two planks, and two horseshoe-shaped wood pieces formed the prow and stern. These were "sewn" together with dowels and lashings. They had no central rudders but were instead steered using an oar on one side. The ancestral rig was the mastless triangular crab claw sail which had two booms that could be tilted to the wind. These were built in the double-canoe configuration or had a single outrigger on the windward side. In Island Southeast Asia, these developed into double outriggers on each side that provided greater stability. The triangular crab claw sails also later developed into square or rectangular tanja sails, which like crab claw sails, had booms spanning the upper and lower edges. Fixed masts also developed later in both Southeast Asia (usually as bipod or tripod masts) and Oceania.

Early researchers like Heine-Geldern (1932) and Hornell (1943) once believed that catamarans evolved from outrigger boats, but modern authors specializing in Austronesian cultures like Doran (1981) and Mahdi (1988) now believe it to be the opposite.

Two canoes bound together developed directly from minimal raft technologies of two logs tied together. Over time, the double-hulled canoe form developed into the asymmetric double canoe, where one hull is smaller than the other. Eventually the smaller hull became the prototype outrigger, giving way to the single outrigger canoe, which diverged into the reversible single outrigger canoe in Oceania. Finally, the single outrigger types developed into the double outrigger canoe (or trimarans).