"It Ain't Necessarily So" is a popular song with music by George Gershwin and lyrics by his brother Ira Gershwin. The song comes from the Gershwins' opera Porgy and Bess (1935) where it is sung by the character Sportin' Life, a drug dealer, who expresses his doubt about several statements in the Bible. The song's melody also functions as a theme for Sportin' Life's character.

Porgy and Bess is an English-language opera by American composer George Gershwin, with a libretto written by author DuBose Heyward and lyricist Ira Gershwin. It was adapted from Dorothy Heyward and DuBose Heyward's play Porgy, itself an adaptation of DuBose Heyward's 1925 novel Porgy.