Marilyn Jean Buck (December 13, 1947–August 3, 2010) was an American Marxist, feminist poet, and anti-war, anti-imperialist, and anti-racist activist, who was imprisoned for her participation in the 1979 prison escape of Assata Shakur, the 1981 Brink's robbery, and the 1983 U.S. Senate bombing. Buck received an 80-year sentence, which she served in federal prison, from where she published numerous articles as well as poetry. She was released on July 15, 2010, less than a month before her death at age 62 from cancer.

Buck was born December 13, 1947, in Temple, Texas, to Virginia Buck, a nurse, and Louis Buck, an Episcopal minister. Her family was active in the civil rights movement. Louis Buck opposed segregation at St. Andrew's Episcopal School in Austin, Texas, picketed, and harshly criticized the bishop. In response, crosses were burned on the family's lawn, and he was removed as minister from the congregation of St. James in Austin, a congregation which had been integrated by the previous clergyman and his family. Louis Buck returned to his veterinarian career, from which he had entered the clergy, to support his family.

Marilyn Buck attended the University of California at Berkeley and the University of Texas at Austin. She subsequently earned a master's degree in Poetics from New College.

At the University of Texas, Buck was involved in organizing against the Vietnam War, as well as anti-racist activities. She joined Students for a Democratic Society (SDS) and worked with Austin's underground newspaper, The Rag. In 1967, Buck moved to Chicago where she edited SDS' New Left Notes and attended an SDS teacher-organizer school. With other SDS women she helped to incorporate women's liberation into the organization's politics. She subsequently returned to San Francisco where she worked with Third World Newsreel in outreach in support of Native American and Palestinian sovereignty and against U.S. intervention in Iran and Vietnam and in solidarity with the Black liberation movement. With colleague Karen Ross, she explained their practice: "We stop people on the street, and confront them with our films. Involve them as participants. It has come to them during a walk down the street, they’ve stumbled upon it. They have been confronted. The decision to watch, to register disgust or interest is now theirs. To those inquisitive, we explain more."

In 1973, Buck was convicted on two counts of purchasing (otherwise legal) ammunition using false identification and sentenced to ten years in prison. In 1977 Buck was given a furlough from prison and went underground instead of returning.

In 1979, Assata Shakur, who had been convicted of killing a policeman, escaped from a New Jersey prison with help from a number of associates outside. In 1983, Buck was recaptured and convicted of participating in Shakur's escape.

Along with a number of Black Liberation Army members and supporters, Buck was convicted of conspiracies to commit armed robbery in the Brinks robbery of 1981, in which a guard and two police officers were killed. She allegedly drove one of the getaway cars, as well as helping to obtain a safe house and weapons. During the investigation into the armed robbery and killings, investigators found weapons and papers in an apartment in East Orange, New Jersey, rented by "Carol Durant," an alias of Buck.

Papers there led police to an address in Mount Vernon, New York, where they found bloody clothing and ammunition belonging to Buck.