"Wouldn't It Be Nice" is a song by the American rock band the Beach Boys and the opening track from their 1966 album Pet Sounds. Written by Brian Wilson, Tony Asher, and Mike Love, it is distinguished for its sophisticated Wall of Sound-style arrangement and refined vocal performances, and is regarded among the band's finest songs. With its juxtaposition of joyous-sounding music and melancholic lyrics, it is considered a formative work of power pop, and with respect to musical innovation, progressive pop.

The song was inspired by Wilson's confused infatuations for his sister-in-law, who projected an "innocent aura" that he wished to capture in "Wouldn't It Be Nice". Lyrically, the song describes a young couple who feel empowered by their monogamous relationship and fantasize about the romantic freedom they would earn as adults. Like the other tracks on Pet Sounds, it subverted listeners' expectations, as past Beach Boys songs had normally celebrated superficial conceits such as material possessions and casual flings.

Wilson produced the record between January and April 1966 with his band and 16 studio musicians who variously played drums, timpani, glockenspiel, trumpet, saxophones, accordions, guitars, pianos, and upright bass. The harp-like instrument heard in the introduction is a 12-string mando-guitar plugged directly into the recording console. One section of the song engages in a ritardando, a device that is rarely used in pop music. The band struggled to sing the multiple vocal parts to Wilson's satisfaction, and the song ultimately took longer to record than any other track on the album.

"Wouldn't It Be Nice" was released as a single in July and peaked at number 8 on the Billboard Hot 100. It has occasionally appeared in the soundtracks of films such as the 1989 documentary Roger & Me, where it was used to underscore visuals of economic devastation. As of 2021, it was the band's most streamed song on Spotify, and it is listed among the thousand-most highest rated songs of all time on Acclaimed Music.