Sydney Greenstreet (December 27, 1879 - January 18, 1954) was an actor.

Born in Sandwich, England, he worked as a comedian as a child, then emigrated to America and performed on the stage, making his stage debut in 1902.

Greenstreet did not act in film until 1941, when he began working for Warner Bros. at the age of 61. He is perhaps most famous for his first film role, that of Kasper Gutman (aka "The Fat Man") in The Maltese Falcon. However, he is also remembered for his role as Signor Ferrari, the sleazy bar owner, in Casablanca (1942). He appeared in several other Warners pictures, typically playing villains.

He died from diabetes in 1954. He is interred in Forest Lawn Memorial Park Cemetery, in Glendale, California.

He is the great uncle of actor Mark Greenstreet.