Dan Marino -- When In London ... Play Catch With Prince Harry!

Dan Marino found a brand new wide receiver in London today ... Prince Harry of Wales ... who fired around the pigskin with the NFL legend during a charity event in the U.K.

Marino is across the pond with the Miami Dolphins ... who play the NY Jets at Wembley Stadium on Sunday.

But before the game, he hooked up with Prince Harry to participate in an event called Walking with the Wounded's Walk of Britain -- where injured military veterans walk 1,000 miles in 72 days to raise money and awareness to soldiers injured during their service.

During the walk, the two stars found time to play catch -- and we gotta say, Harry's form didn't look too bad for a guy who's never strapped on the pads.

Side note -- nice beard Harry ... but you're no Ryan Fitzpatrick.