West Side Story is a musical written by Arthur Laurents (book), Leonard Bernstein (music), and Stephen Sondheim (lyrics). The story is based loosely on Shakespeare's Romeo and Juliet, which was, in turn, based on a narrative poem by Arthur Brooke.

Set on Manhattan's Upper West Side, the musical explores the rivalry between two teenage gangs of different ethnic and cultural backgrounds. The young protagonist, Anton ("Tony"), who belongs to the white gang, falls in love with Maria, the sister of the leader of the rival Puerto Rican gang. The dark theme, sophisticated music, extended dance scenes and focus on social problems marked a turning point in American musical theater. Bernstein's score for the musical has become extremely popular, including "Something's Coming", "Maria", "America," "Somewhere," "Tonight", "Jet Song", "I Feel Pretty", "One Hand, One Heart", and "Cool".

The original 1957 Broadway production, directed and choreographed by Jerome Robbins and produced by Robert E. Griffith and Harold Prince, marked Stephen Sondheim's Broadway debut. It ran for 732 performances (a successful run for the time), before going on tour. The production garnered a Tony Award nomination for Best Musical in 1957.

The show has enjoyed an even longer-running London production, a number of revivals and international success, and spawned an innovative, award-winning 1961 musical film of the same name. West Side Story is produced frequently by local theaters and, occasionally, by opera companies.