Frankie Muniz has McDreamy in his corner ahead of his full-time gig with the NASCAR Truck Series ... 'cause Patrick Dempsey tells TMZ Sports he hopes his fellow actor/racer crushes it!!

The 38-year-old former "Malcolm in the Middle" star -- who got into motorsports after his time in front of the camera -- was announced as the driver of the No. 33 Ford for Reaume Brothers Racing last week ... and, will have a full-time seat for 2025.

Dempsey couldn't be more stoked for Muniz ... telling TMZ Sports he's been closely following the guy's career on the track -- and, he's got what it takes to make some noise.

"Frankie Muniz has gotta ride for the entire season," the former World's Sexiest Man said in NYC. "He's an incredible driver. I'm really happy for him. His last race was really positive. I wish him the best of luck."

Dempsey knows his stuff -- he even started Dempsey Racing in 2006, which helps new talent learn the ropes from some of the top professionals across the globe.