Marthe Keller (born 28 January 1945; Basel, Switzerland) is a Swiss actress and opera director. She studied ballet as a child, but stopped after a skiing accident at age 16. She changed to acting, and worked in Berlin at the Schiller Theatre and the Berliner Ensemble.

Keller's earliest film appearances were in Funeral in Berlin (1966, uncredited) and the German film Wilder Reiter GmbH (1967). She appeared in a series of French films in the 1970s, including Un cave (1971), La raison du plus fou (1973) and Toute une vie (And Now My Love, 1974). Her most famous American film appearances are her Golden Globe-nominated performance as Dustin Hoffman's girlfriend in Marathon Man and her performance as a femme fatale Arab terrorist who leads an attack on the Super Bowl in Black Sunday, both of them were ill-fated characters at the climax of each film. Keller also acted with William Holden in the 1978 Billy Wilder film Fedora. She appeared alongside Al Pacino in the auto racing film Bobby Deerfield, and subsequently the two of them were involved in a relationship. Since then, Keller has worked more steadily in European cinema compared to American movies. Her later films include Dark Eyes, with Marcello Mastroianni.