Robert Allen Wiethoff (born September 15, 1976) is an American actor and voice actor. He is best known for portraying John Marston in the 2010 video game Red Dead Redemption, its expansion Undead Nightmare, and the 2018 prequel Red Dead Redemption 2.

Born and raised in Seymour, Indiana, Wiethoff attended Seymour High School, and graduated from Indiana University Bloomington in 1999. Soon afterwards, he moved to Los Angeles to pursue acting, but faced difficulty in finding roles, other than small parts in films and television advertisements. In 2008, Rockstar Games cast Wiethoff as John Marston, the lead character in Red Dead Redemption; he worked on the game for almost two years.