A flying boat is a type of fixed-winged seaplane with a hull, allowing it to land on water. It differs from a floatplane in having a fuselage that is purpose-designed for floatation, while floatplanes rely on fuselage-mounted floats for buoyancy. It differs from an amphibious aircraft by lacking wheels, skis, or skids to land on a solid surface rather than water.

Though a flying boat’s fuselage provides buoyancy, it may also utilize under-wing floats or wing-like hull projections (called sponsons) for additional stability.

Ascending into common use during the First World War, flying boats rapidly grew in both scale and capability during the interwar period, during which time numerous operators found commercial success with the type. Flying boats were some of the largest aircraft of the first half of the 20th century, exceeded in size only by bombers developed during the Second World War. Their advantage lay in using water instead of expensive land-based runways, making them the basis for international airlines in the interwar period. They were also commonly used as maritime patrol aircraft and air-sea rescue, particularly during times of conflict. Flying boats such as the PBY Catalina and Short Sunderland played key roles in both the Pacific Theater and the Atlantic of the Second World War.

The popularity of flying boats gradually trailed off during the Cold War era, partially because of the difficulty in maintaining operations in inclement weather when sea states may easily prevent taking off and landing while land based aircraft are unaffected, and investments in airports during the conflict that eased the introduction of larger, and more efficient, land-based airliners. Despite being largely overshadowed, limited use of the type continued with some operators, such as in the case of the Shin Meiwa US-1A and the Martin JRM Mars. In the 21st century, flying boats maintain a few niche uses, such as dropping water on forest fires, air transport around archipelagos, and access to undeveloped areas. Many modern seaplane variants, whether float or flying boat types, are convertible amphibious aircraft where either landing gear or flotation modes may be used to land and take off.