Last Christmas, released later this year, is set to star Emilia Clarke, Henry Golding and Emma Thompson.
Its director Paul Feig told BBC News: "[George Michael] was putting together his new album when he passed.
"And one of our tracks is one of those songs, and it's just an absolutely amazing song that I'm so excited the world is going to get to hear now."
George Michael was found dead on Christmas Day in 2016 at his home in Goring, Oxfordshire.
Last Christmas focuses on a young aspiring singer (played by Clarke), whose hero is George Michael. Feig explains the singer's music "sort of travels with her as she goes through her life" in the storyline.
Speaking about the new track, he continued: "It's a very celebratory song, I would dare say. And we were able to play the entire song, which is almost six minutes long, in the film.
"Because when you get a song that has never been heard, you don't want to just use, like, 15 seconds of it. The song starts at the end of the film, and then goes into the credits."
Last Christmas is set to be released next July (just kidding, it's out in November) and also stars Henry Golding and Michelle Yeoh, both of whom also appeared together in Crazy Rich Asians.
The first trailer for the film premiered on Wednesday morning.