"Sleepy Lagoon" is a popular song, based on an orchestral 'valse serenade' originally written by British composer Eric Coates, with lyrics by Jack Lawrence.

Lawrence heard the music and wrote a song lyric, then took the lyric to Chappell, the publisher of Coates' original melody. The head of Chappell's New York office, Max Dreyfuss, was concerned that this lyric had been added without consulting the composer, who was a famous British classical composer and "may resent your tampering with his melody." Dreyfuss also didn't think the melody belonged in the popular genre, and was better suited to treatment as a light classical piece.

After some time during which Lawrence attempted to contact Coates (this was 1940, and Britain was in the middle of World War II, making communication difficult) communication was established, and contrary to Dreyfuss' fears, Coates thought the lyric fitted so well that one could hardly believe it had been written to a pre-existing melody. The resulting song was published as a collaboration of Lawrence and Coates, and when Lawrence showed the song to bandleader Harry James, it was recorded for a major hit (again counter to Dreyfuss' thoughts). Other hit versions were recorded by Dinah Shore, David Rose, Fred Waring, Glenn Miller and others.

The recording by James was released by Columbia Records as catalog number 36549. It first reached the Billboard magazine Best Seller chart on April 17, 1942 and lasted 18 weeks on the chart, peaking at #1.

In 1942, the original version was recorded (with added seagulls) for introducing the BBC radio series Desert Island Discs, and the theme is still in use today.

The song made the Billboard Hot 100 in 1960, in a version by the Platters, found originally on the flipside of the 1960 top ten "Harbor Lights".

The song is used at one of the rides (The Magic Clock) in De Efteling in The Netherlands, one of Europe's largest theme parks.