Jack Harlow and Doja Cat walk into a bar -- but you haven't seen this story before.

The two superstars just released "Just Us," their first-ever collab after years of teasing, and the video looks like a scene out of a Hollywood bar on Oscar night ... with tons of celeb cameos!!!

Veteran director Neal Farmer rounds up an ensemble cast inside L.A. hot spot Horses ... Jack's "The Instigator" costar Matt Damon, John Mayer, PinkPantheress, Nicholas Braun, and one of Jack's "best friends in the whole world," Taylor Rooks, all show their beautiful mugs.

Malcolm Todd, Jack's Generation Now execs DJ Drama and Leighton Morrison, Anok Yai, Molly Banz and Ben Willett all round out the special cast.

JH's been dropping off loosie tracks for months as he gears up for his next album mode ... Doja just may have helped serve up his next chart-topper!!!