Theodore Nicholas Gill (March 21, 1837 – September 25, 1914) was an American ichthyologist, mammalogist, malacologist and librarian.Born and educated in New York City under private tutors, Gill early showed interest in natural history. He was associated with J. Carson Brevoort in the arrangement of the latter's entomological and ichthyological collections before going to Washington D.C. in 1863 to work at the Smithsonian Institution.