The track is called "Wolves," and it's dropping on Oct. 25.
The singer teased the song with a couple photos of herself with the DJ on Twitter Thursday night.
"Selenators, meet the mellogang," she captioned one. Marshmello tweeted a picture as well, writing, "Coming soon...the newest member of the mellogang."
In an interview over the summer, Gomez described "Wolves" as "beautiful" and "one of my favorite songs" from her upcoming album.
"It's really cool. It's kind of in that world, his world, and I'm stepping into it and bringing my style too," she said.