The 28-year-old singer was widely praised for her tribute to The Sound of Music at the 87th Academy Awards in February, but the acclaimed 84-year-old composer told The London Times he hated the medley.
"She was a travesty," Sondheim declared of the pop star. "It was ridiculous, as it would be from any singer who treats that music in semi-operatic style. She had no relationship to what she was singing. What people liked was her versatility."
Sondheim was mentored by The Sound of Music lyricist Oscar Hammerstein II, and also slammed NBC's production of The Sound of Music Live! as "embarrassingly bad." He himself is known for writing the lyrics to West Side Story, and composed Sweeney Todd and Into the Woods.
Lady Gaga released her third record, Artpop, in November 2013, and last debuted duets album Cheek to Cheek with Tony Bennett in September 2014. The singer and actor Taylor Kinney announced their engagement in February, and Bennett will sing at the couple's wedding.