"Forty Days and Forty Nights" is a blues song recorded by Muddy Waters in 1956. Called "a big, bold record", it was a hit, spending six weeks in the Billboard R&B chart, where it reached number seven. "Forty Days and Forty Nights" has been interpreted and recorded by a variety of artists.

"Forty Days and Forty Nights" is a midtempo blues song with an irregular number of bars written by Bernard Roth (who also wrote Muddy Waters' "Just to Be with You"). An early review called it "a dramatic piece of material with effective lyrics".

Backing Muddy Waters (vocals) are Little Walter (harmonica), Willie Dixon (bass), possibly Fred Below or Francis Clay (drums), Pat Hare (guitar), and Jimmy Rogers or Hubert Sumlin (second guitar). The song was recorded during Pat Hare's first recording session with Waters and "Hare's crunching power chords rippled with distortion that was well suited for blues in the rock and roll explosion".

The song was one of Waters' last charting singles and appears on several of his compilation albums, including the 1965 album The Real Folk Blues. He later recorded "Forty Days and Forty Nights" for the 1969 Fathers and Sons album and the Authorized Bootleg: Live at the Fillmore Auditorium November 4–6, 1966 album released in 2009.

"Forty Days and Forty Nights" has been recorded by various blues and other artists, including Steppenwolf from Steppenwolf 7, John Hammond, Jr., Eric Burdon from his Soul of a Man album, and Buddy Guy for the Cadillac Records soundtrack.

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