Elizabeth Josephine "Jo" Sullivan Loesser is an American soprano and the widow of composer Frank Loesser. Her daughter, Emily Loesser, is also a singer-actress.

Born in Mounds, Illinois, Jo Sullivan played Polly Peachum in Marc Blitzstein's legendary English-language adaptation of The Threepenny Opera by Kurt Weill and Bertolt Brecht off-Broadway in 1954. On Broadway, she starred in her future husband's masterpiece The Most Happy Fella (1956) as "Rosabella", alongside Robert Weede and Susan Johnson, which earned her a 1957 Tony Award nomination as Best Featured Actress in a Musical.