A flying boat is a fixed-winged seaplane with a hull, allowing it to land on water. It differs from a float plane as it uses a purpose-designed fuselage which can float, granting the aircraft buoyancy. Flying boats may be stabilized by under-wing floats or by wing-like projections (called sponsons) from the fuselage. Flying boats were some of the largest aircraft of the first half of the 20th century, superseded in size only by bombers developed during World War II. 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 for maritime patrol and air-sea rescue.

Following World War II, their use gradually tailed off, partially because of the investments in airports during the war. In the 21st century, flying boats maintain a few niche uses, such as for dropping water on forest fires, air transport around archipelagos, and access to undeveloped or roadless areas. Many modern seaplane variants, whether float or flying boat types are convertible amphibian aircraft where either landing gear or flotation modes may be used to land and take off.