Dame Agnes Sybil Thorndike, Lady Casson (24 October 18829 June 1976), was an English actress whose stage career lasted from 1904 to 1969.

Trained in her youth as a concert pianist, Thorndike turned to the stage when a medical problem with her hands ruled out a musical career. She began her professional acting career with the company of the actor-manager Ben Greet, with whom she toured the US from 1904 to 1908. In Britain she played in old and new plays on tour and in the West End,