"I Know There's an Answer" is a song by American rock band the Beach Boys, the 9th track on their 1966 album Pet Sounds. It was composed and produced by Brian Wilson in lyrical collaboration with bandmate Mike Love and the group's road manager Terry Sachen. The song was written as a reaction to Wilson's experiences with lysergic acid diethylamide (LSD), as he explained, "I had gotten into that kind of thing. I guess it just came up naturally."

It is characterized by its unusual juxtaposition of instrumentation which unfolds as the song progresses, which include tack piano, bass harmonica, banjos, saxophones, flutes, and organ.MOJO described it: "A fried treatise on how LSD separates the turned-on 'us' from the uptight 'them'." For a 1990 CD reissue of Pet Sounds, an earlier recording of the song with different lyrics was released as a bonus track under the name "Hang On to Your Ego".

Love was not originally listed as co-writer, and only received his credit after a 1990s court case. During the song's recording sessions, conflicts arose between Wilson and his bandmate. Love thought it was "too much of a doper song" and suggested that it should instead be about "finding yourself". After careful consideration, Wilson allowed Love to revise some of the song's lyrics, renaming it from "Hang On to Your Ego" to "I Know There's an Answer". The song's references to LSD were ultimately kept in its lyrics.

Its composition features a verse/refrain/verse/refrain/bridge/refrain pattern, music framing devices consistent with other tracks of Pet Sounds such as "You Still Believe in Me", along with a melody which stretches two octaves. Dissimilar to other tracks is its key movement which occurs for the lyric "now what can you tell them"—it proceeds up a minor third whereas Wilson usually proceeded down.Inverted chords are used just as they are in other Pet Sounds compositions.

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