Roy Marcus Cohn (February 20, 1927 - August 2, 1986) was an American lawyer who came to prominence during the investigations by Senator Joseph McCarthy into communism in the government and especially during the Army McCarthy hearings. He was widely unpopular during his lifetime but he wielded tremendous political power locally.

During his lifetime he denied the rumors that he was gay. He dated Barbara Walters who publicly announced that she thought he was heterosexual. When he died it was initially announced that he had died of liver cancer: in fact he had died of AIDS. The hypocrisy of his life as a homophobic homosexual was so emblematic that he became a character in two of Tony Kushner's plays Angels in America, and G. David Schine in Hell. Actor Al Pacino portrayed Cohn in the production of Angels in America, also starring Justin Kirk, Meryl Streep, Ben Shenkman, Patrick Wilson, Emma Thompson and Mary Louise Parker, which aired on HBO in 2003.

Cohn's father was an influential New York judge. Cohn graduated from Columbia Law School at the age of 20 and began work for the United States Attorney in Manhattan. He prosecuted Julius and Ethel Rosenberg, and his cross-examination of Ethel's brother produced the testimony that was mostly responsible for their conviction and execution.

As McCarthy's henchman and chief counsel he was responsible for grilling suspected communists on their own sexual tendencies and on whether other people had "homosexual tendencies". Cohn and McCarthy subpoenaed gay men in the arts and threatened to out them if they did not produce a list of "suspected Communists". McCarthy probably did not know Cohn was gay.

In 1954, when McCarthy began to look for subversives in the Army, President Dwight David Eisenhower arranged to have the hearings televised so the public would see McCarthy's bullying tactics. As a result of the public outrage the Senate censured McCarthy in December 1954.

In the early 1980s Cohn was disbarred on grounds of unethical and unprofessional conduct.