Frank Knox Morton Rehn (April 12, 1848 – July 7, 1914) was an American marine painter, and president of the Salmagundi Club.

[1] Born in Philadelphia, he attended the Pennsylvania Academy of the Fine Arts, where he studied under Christian Schussele.

[2] Using money earned in Philadelphia, he moved to the coast of New Jersey, where he began doing marine paintings.

They moved to the Hotel Chelsea in New York City where, with other artists, he kept a studio on the top floor.

[1][4] A son, also named Frank Knox Morton Rehn (1886–1956), owned a well-known Manhattan art gallery; among the artists he represented was Edward Hopper.