The Hansa-Brandenburg W.20 was an experimental submarine-launched maritime reconnaissance flying boat designed by the Hansa-Brandenburg Aircraft Company () for the Imperial German Navy's () Naval Air Service () during World War I. Three prototypes were completed in 1918, but the W.20 was not put into production.
Due to the need to be stored and launched from a submarine aircraft carrier, the W.20 was a small single-seat, pusher configuration, biplane flying boat that was designed to be assembled and dismantled quickly. It was powered by a seven-cylinder, Oberursel U.0 rotary engine mounted on struts between the wings driving a two-bladed propeller. The pilot had an open cockpit just forward of the lower wing. Stabilising floats were fitted on the lower wings. The W.20 was unarmed.
Three prototypes were ordered on 30 April 1917 as well as a full-scale mockup to test the aircraft's ability to fit within the cylinder that was going to be mounted on top of the submarine's hull. The prototypes were built using the company's standard wooden fuselage and fabric-covered wooden wings to prove the feasibility of the concept, but production aircraft were intended to be of all-metal construction. The initial design of the structure supporting the upper wings proved too weak and the first prototype was severely damaged in October 1917. Additional interplane struts connecting the upper wings with the lower ones had to be added and the existing struts were strengthened. In addition, the wings were lengthened. The second prototype was fitted with these modifications and delivered in March 1918 to the Seaplane Experimental Command () and flight tested the following month. It could not complete the trials, but it was accepted by the Naval Air Service anyway. After the required modifications and repairs were made to the aircraft, all three were sent into storage at Warnemünde. All three W.20s were found at Hage when the Allies inspected the German seaplane bases in December 1918. Their ultimate fate is unknown, but it is likely that they were scrapped.