Eilish surprised Keys with the clip on Monday as the R&B star was serving as guest host of the late night talk show.
A younger and blonde haired Eilish is featured in the video singing "Fallin" while playing the piano. Keys was impressed with the performance and called it amazing.
Eilish presented the video after Keys had released on Instagram a video of herself covering the singer's song "Ocean Eyes."
The pair later teamed up for a duet of "Ocean Eyes" with Keys using a piano located inside the Late Late Show desk.
Keys is set to host the 62 annual Grammy Awards on Jan. 26. Eilish is nominated for six awards including Album of the Year for When We All Fall Asleep Where Do We Go?, Record of the Year for "Bad Guy" and Best New Artist.