Leo John Genn (9 August 1905 – 26 January 1978) was an English actor and barrister. Signified by his relaxed charm and smooth, "black velvet" voice, he had a lengthy career in theatre, film, television, and radio; often playing aristocratic or gentlemanly, sophisticate roles.

Born to a Jewish family in London, Genn was educated as a lawyer and was a practicing barrister until after World War II, in which he served in the Royal Artillery as a Lieutenant-Colonel. He began his acting career at The Old Vic and made his film debut in 1935, starring in a total of 85 screen roles until his death in 1978. For his portrayal of Petronius in the 1951 Hollywood epic Quo Vadis, he received an Oscar nomination for Best Supporting Actor.