Enter Sir John is a 1928 British crime novel by Clemence Dane and Helen Simpson.

It concerns Martella Baring, a young actress, who is put on trial and convicted of murder and a fellow actor Sir John Saumarez who takes up her cause and tries to prove her innocence.

Young actress Martella Baring is convicted of the murder of Edna Druce, the wife of the acting company's manager.

The charming and clever Sir John Saumarez, himself an actor and the manager of an acting company, attends the trial and becomes convinced of Martella's innocence.

[2] A number of changes were made from novel to screen, such as making Sir John a member of the jury while in the book he was just a spectator at the trial.