John Travolta Dances Onstage with 50 Cent at Cannes

John Travolta got a case of "Saturday Night Fever" on a weeknight in Cannes, and showed off his dancing skills right next to 50 Cent ... well, just a lil bit.

The "Grease" and "Staying Alive" actor/dance legend joined the rapper onstage in a full tuxedo ... and busted out some very limited moves for the crowd. The outfit doesn't really scream hip-hop, but was pretty fitting for the dad vibe he gave off.

Travolta's at the annual film fest for the premiere of "Gotti" -- which he produced and stars in as the notorious Gambino family crime boss.

We haven't seen it yet, but we've seen all his dance flicks and we'll just say this -- he's still got his rhythm.