"Mannish Boy" (or "Manish Boy" as it was originally titled) is a blues standard by Muddy Waters. First recorded in 1955, the song is both an arrangement of and an "answer song" to Bo Diddley's "I'm a Man", which was in turn inspired by Waters' and Willie Dixon's "Hoochie Coochie Man". "Mannish Boy" features a repeating stop-time figure on one chord throughout the song and is credited to Waters, Mel London, and Bo Diddley.

The original version of "Mannish Boy" was recorded in Chicago on May 24, 1955, under the title "Manish Boy." Accompanying Muddy Waters were Jimmy Rogers on guitar, Junior Wells on harmonica, Fred Below on drums, and an un-identified female chorus. The original version was the only recording done by Muddy Waters between January 1953 and June 1957 that did not feature Little Walter on harmonica (who was on tour supporting his then-number one hit "My Babe" and thus unavailable for the recording session) and was one of few studio recordings with Junior Wells.

Muddy Waters recorded several versions of "Mannish Boy" during his career. In 1968, he recorded it for the Electric Mud album in Marshall Chess' attempt to attract the rock market. After he left Chess, he recorded it for the 1977 Hard Again album which was produced by Johnny Winter. The song also was included on the live album Muddy "Mississippi" Waters - Live (1979). He also performed it at The Band's farewell concert The Last Waltz, and the performance was included in the documentary film of the concert as well as on the film's soundtrack of the same title.

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