"Once Upon a Long Ago" is a Paul McCartney song, released as his fortieth single on 16 November 1987, as part of his compilation All the Best!, released two weeks before the single. The track was produced by Phil Ramone and mixed by George Martin, and features violin by Nigel Kennedy.

It was reported in a 2011 biography that the origin of this song is with the film The Princess Bride. McCartney was approached by Director Rob Reiner to compose a couple of songs and the incidental music for the film. Reiner rejected as "too sentimental" the two songs Paul submitted: "Once Upon a Long Ago" and "Beautiful Night" and contracted with Mark Knopfler to complete the soundtrack.

Originally it was intended that this song would be a Duet between McCartney and Freddie Mercury. The two had become friends following Live Aid and McCartney wrote the song with Mercury in mind. Mercury however was inundated with work, both Solo and with Queen and was also in the advanced stages of AIDS and McCartney continued the project without him. The track was recorded at Hog Hill Studio on March 11 and 12, 1987 with overdubs added in Abbey Road in July. During this session, a full orchestra was overdubbed as well as instrumental contributions by Stan Sulzmann (saxophone), Nigel Kennedy (violin), and Adrian Brett (flute).

The song was released in four versions. Two different 12" singles feature "long" and "extended" versions (mixed by George Martin and Peter Henderson, respectively); the 7" single and the CD single (McCartney's first) features an edit of the long version and a b-side from each the two 12" singles, as well as "Back on My Feet", a b-side for all four iterations; and the album version featured on All the Best! contains an alternative ending. "Back on My Feet" was also the first released song from the songwriting collaboration between McCartney and Elvis Costello. Both 12" and CD singles also featured songs from yet to be released McCartney's cover album Choba B CCCP.

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