I Don't Wanna Live Forever is Swift's first release since her multi million-selling 1989 album.
She shared news of the single with her 82 million Twitter followers with the cryptic comment: "Z T 50".
The announcement was retweeted nearly 15,000 times in an hour; while her Instagram post, featuring a snippet of the song, has been viewed 2m times.
A yearning, sultry ballad, I Don't Wanna Live Forever finds the couple pulling at the unravelling threads of a relationship.
"Wonder if I dodged a bullet / Or just lost the love of my life?" sings Swift, before hitting a breathy falsetto for the chorus.
"What is happening to me? / I don't wanna live forever/ 'Cause I know I'll be living in vain," responds Zayn. "I just want to keep calling your name, until you come back home."
Produced by fun.'s Jack Antonoff, the song was co-written by Antonoff, Swift and Sam Dew.
The single has been posted on iTunes and Apple Music but is currently not available on other streaming or download services.
The song doubles as Malik's final release of 2016. He recently confirmed his next song would be the first single from his upcoming second album, due out in 2017.
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Pop stars Taylor Swift and Zayn Malik have surprised fans by recording a duet for the Fifty Shades Darker soundtrack.