Theodore Case Sound Test: Gus Visser and his Singing Duck, also known as Gus Visser and His Singing Duck, is a 1925 American short musical comedy film starring vaudeville performer Gus Visser. The short is an early sound film, directed by Theodore Case while perfecting his variable density sound-on-film process. Case began working on his sound film process at the Case Research Lab in Auburn, New York, in 1921.

In 2002, the film was selected for the National Film Registry at the Library of Congress as a historically important American film. A print of the film is preserved at the George Eastman House.