Dame Sybil Thorndike (October 24, 1882 - June 9, 1976) was a British actress.

She was born in Gainsborough, Lincolnshire, trained at Ben Greet's Academy, and was on stage from 1904. In 1908 she met George Bernard Shaw, and at around the same time met Lewis Casson, who soon became her husband. They had four children. Although she appeared in several of Shaw's plays, it was not until 1933 that he began work on his play, Saint Joan, intending the leading role for Sybil Thorndike. Like Shaw, Thorndike and Casson were both politically left-wing.

Among Sybil Thorndike's other famous roles were Medea and Lady Macbeth. She was best known as a stage actress, though she appeared in several films. She was made a Dame in 1931. The Thorndike Theatre in Leatherhead, Surry , opened in 1969, the year Sir Lewis Casson died.

A biography by Sheridan Morley was published in 1977.