Otto James Messmer (/ˈmɛzmər/; August 16, 1892 – October 28, 1983) was an American animator known for his work on the Felix the Cat cartoons and comic strip produced by the Pat Sullivan studio.

The extent of Messmer's role in the creation and popularity of Felix is a matter of ongoing dispute, particularly as he only laid his claim to the character after the death of Sullivan, who until that time had received the credit.

He attended the Thomas School of Art in New York City from 1911–1913, and participated in a work-study program with the Acme Agency, where he did illustrations for fashion catalogs.

Inspired by Winsor McCay's animated films, such as How a Mosquito Operates, Messmer began creating his comics for local newspapers in 1912, the same year he met Anne Mason, whom he married in 1934.

It was never released, but drew the interest of animator Pat Sullivan, though Messmer instead decided to go to work with Henry "Hy" Mayer, a well-known cartoonist.

Sullivan gave the project to Messmer, whose end result, Feline Follies, starred Master Tom, a black cat, who was a prototype to Felix, which brought good luck to people in trouble.