Jeff 'Swampy' Marsh (born 9 December 1960) is an American animator, writer, director, producer, and voice actor who has worked on several animated television series, best known as executive producer and the voice of Major Monogram in the Disney animated series Phineas and Ferb and Milo Murphy's Law, which he co-created with Dan Povenmire. Marsh was born in Santa Monica, California, where he grew up in a very mixed family environment.

Marsh was and remains a driving force behind several animation projects, having worked for more than six seasons on the animated television series The Simpsons. Marsh continued to work on other animated television series, including King of the Hill and The Life of Modern Rocco, before moving to England in 1996.