"Wouldn't It Be Nice" is a song written by Brian Wilson, Tony Asher, and Mike Love for American rock band the Beach Boys, released as the opening track on their 1966 album Pet Sounds. The song was also released as a single two months after the album's release with "God Only Knows" as its B-side. In other countries, the sides were flipped, with "Wouldn't It Be Nice" as the single's B-side. Its lyrics describe a couple in love lamenting about being too young to run off to get married, fantasizing about how nice it would be if they were adults.

Like other tracks for Pet Sounds, Wilson constructed the song's symphonic Wall of Sound arrangement using a variety of instruments not normally associated with popular music of its time, including accordions and a detuned twelve-string guitar. The music contains classical music devices that are unusual for a rock song, such as bitonality and ritardando. After recording the instrumental track, the Beach Boys overdubbed their voices to Wilson's exact specifications. Love was not originally listed as co-writer, and only received his credit after a 1990s court case.

In 2006, Pitchfork Media placed "Wouldn't It Be Nice" at number 7 on its list of "The 200 Greatest Songs of the 1960s".

Tony Asher credits the song's idea to Brian Wilson: "The innocence of the situation — being too young to get married — seemed to be immensely appealing to him." Wilson composed the music, while the lyrics were written almost entirely by Asher over the course of one or two days. Mike Love's single contribution was the ending couplet "Good night my baby / sleep tight my baby".

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