Mildred Miller Posvar (December 16, 1924 – November 29, 2023) was an American classical mezzo-soprano who had a major career performing in operas, concerts, and recitals during the mid twentieth century. She was notably a principal artist at the Metropolitan Opera from 1951 through 1974. In 1978 she founded the Opera Theater of Pittsburgh, serving as the company's Artistic Director through 1999.

As a performer, Miller won particular acclaim for her singing of German Lieder and recorded with, among others, conductor Bruno Walter, with whom she won a Grand Prix du Disque for Mahler's Lieder eines fahrenden Gesellen. On the opera stage she became known for her portrayal of pants roles (in particular the role of Cherubino in The Marriage of Figaro), garnering the nickname "Legs Miller". She is a Gold Medal recipient from the American Academy of Arts and Letters, and held honorary degrees from the Cleveland Institute of Music, the New England Conservatory, Bowling Green State University, and Washington & Jefferson College.