The acoustic track, titled “Who You Love” – along with its black-and-white cover art featuring the sultry couple – was revealed via John’s Twitter.
He wrote: “‘Who You Love’ is here. Proud of this song, proud of this artwork, proud of this girl. – JM.”
The song is the third single off of the 36-year-old's new album, Paradise Valley. Katy, 29, wrote the lyrics for the part she sings.
"I'm so proud of it because people hear it, and they hear a different side of me,” she said.
John is equally pleased with their lovey-dovey single, previously saying: “I wouldn’t have brought her a song if I didn’t think it was good and great and she wouldn’t have said she would do it if she didn’t think it was going to be great. So it was a completely artistic transaction.”
The musical couple have been dating on-and-off since 2012. Katy had previously been married to comedian Russell Brand before calling it quits in 2011.
Listen to “Who You Love” below.