On September 27, 1933, Igor Sikorsky received U.S. Patent No. 1,879,716 on an amphibian aircraft:

Seaplanes generally are powered fixed-wing aircraft capable of taking off from and landing on water. They are usually divided into floatplanes and flying boats. A float plane is, as the name implies, a plane with float. A flying boat is an aircraft where the body of the craft floats. What makes Sikorsky’s aircraft “amphibian” is that it is capable of taking off from and landing on both land and water.

Glenn Curtiss patented a float plane in 1922:

and a flying boat in 1915: