Joanna Merlin (born Joann Ratner; July 15, 1931) was an American actress and casting director who has worked with Stephen Sondheim and starred in the original Broadway production of Fiddler on the Roof. She has written two acting guides and is a faculty member of New York University's graduate acting program. In recent years, she has become known for her recurring role on Law & Order: Special Victims Unit as Judge Lena Petrovsky.

Joann Ratner, who later took her mother's maiden name, was born in Chicago, Illinois, to Toni Merlin and Harry Ratner, a Jewish grocer. Her family also included her older sister Harriet Glickman (1926-2020) who would go on to gain recognition for having helped persuade Charles Schulz to add a black character to his Peanuts strip. Merlin first acted on stage at age 11, joining a community theatre production, Too Many Marys. Merlin graduated from UCLA and later studied under Michael Chekhov, learning his world-renowned acting technique. She has been described as the last student of Michael Chekhov who is still alive and teaching.