John Percy Lockhart-Mummery FRCS (14 February 1875 – 24 April 1957), was a British surgeon at St Mark's Hospital, London, who devised a classification of rectal cancer and described familial polyposis which led to the formation of the polyposis registry. He was the author of several books, including Diseases of the Rectum and Colon and their Surgical Treatment (1923) and The Origin of Cancer (1934). His work on colorectal surgery earned him the nickname "King Rectum".