Robert John Maudsley (born 26 June 1953) is an English vigilante and serial killer. Maudsley first killed a man who showed him pictures of children he had sexually abused. After surrendering himself to police and saying he needed psychiatric care, Maudsley was sent to Broadmoor Hospital, where he killed a convicted child molester. He later killed two men on the same day: one imprisoned for murdering and sexually assaulting his wife and another imprisoned for attempting to strangle a four-year-old girl. Maudsley's killings have been described as vigilantism.

Maudsley is the longest-serving British prisoner in solitary confinement.