The television personalities, both 38, shared a video together Thursday on Instagram.
"#SecretProject with @KimKardashian," Hilton captioned the clip. "I can't stop looking at my #BestFriendsAss."
The video shows Hilton and Kardashian wearing silver outfits and voguing for the camera.
"You guys know what we're doing today?" Kardashian asks.
"It's a surprise," Hilton says.
TMZ said Hilton and Kardashian were filming a music video for Hilton's new single "Best Friend's Ass." The pair were spotted on set Thursday in Los Angeles with RuPaul's Drag Race alum and Britney Spears impersonator Derrick Barry.
Hilton released her debut studio album, Paris, in August 2006. She last released the single "I Need You" on Valentine's Day in February 2018.