"The Little Girl I Once Knew" is a song written by Brian Wilson for the American rock band the Beach Boys, released as a non-album single in 1965.

AllMusic called the song "a virtual link between the slightly progressive work on songs such as 'California Girls' and the then-quantum leap taken by Wilson on Pet Sounds and 'Good Vibrations'". Unified by an opening hook which intertwines keyboards with guitars, the song is eccentric for using stop-start melody sections along with a few dramatic periods of silence lasting several seconds each.

"The Little Girl I Once Knew" follows familiar territory as the Beach Boys' 1964 outtake "All Dressed Up for School", telling the story of a guy who reacquaints with a girl from his past who has now grown up and catches his eye. According to Mark Dillon, some speculate that the song was written about Brian's then-wife Marilyn Wilson.Mike Love reportedly had a hand in the lyrics. In the 1990s, his lawsuit for officially-recognized writing contributions failed to include "The Little Girl I Once Knew".

The song was recorded shortly after Brian completed Beach Boys' Party! in between sessions for the Pet Sounds tracks "Sloop John B" and "You Still Believe in Me". It was initially labelled "Carol K" on its session tape box, a reference to the song's bassist, Carol Kaye. The track has drawn comparisons with the work of contemporary Burt Bacharach, whom Wilson admired. In 1995, Wilson expressed, "It was a fine song, except the intro is the only good part of it, and the rest didn't sound so good. I thought the song in itself sucked. I didn't like the harmonies, I thought they were sour and off-key." Despite this, he later said that the song "should've" gotten more attention than it did. Its verses are "low-key" and hard-shift into a "blaring chorus", which Dillon writes, "foreshadows the Smile track 'Cabin Essence'".

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