"That's Not Me" is a song written by Brian Wilson and Tony Asher for American rock band the Beach Boys, the third track from their 1966 album Pet Sounds. It is a work influenced by psychedelic drugs that inspired Wilson to turn his attention inward and probe his deep-seated self-doubts. Wilson explained that the song "reveals a lot about myself, just the idea that you're going to look at yourself and say, 'Hey, now look, that's not me, kind of square off with yourself and say 'this is me, that's not me'." It is the only track on Pet Sounds that resembles a conventional rock song.

The song's structure contains multiple key modulations and mood shifts. On its recording, Mike Love shares lead vocal with Wilson, who plays the track's prominent, buzzing organ. Brian's brothers Carl and Dennis also contribute guitar and drums, respectively. Other band members sing backing vocals.

The song was written by Brian Wilson and Tony Asher. While it is commonly understood that Wilson composed the majority of the music on Pet Sounds, it has been claimed in Steven Gaines' book Heroes and Villains that "That's Not Me" was one of three songs in which Asher contributed musical ideas rather than acting solely as a co-lyricist; the other two being "Caroline, No" and "I Just Wasn't Made for These Times".

Asher has expressed dissatisfaction with the lyrics for the song, describing them as "labored", and that he had "missed the mark". In 1976, Brian stated, "I think "That's Not Me" reveals a lot about myself, just the idea that you're going to look at yourself and say, 'Hey, now look, that's not me, kind of square off with yourself and say 'this is me, that's not me'." According to Brian, "Tony and I actually didn't even really realize who we were writing it for. We didn't really write for anybody; we just wrote the songs. So we had full carte blanche to create these songs."

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