Leonardo DiCaprio -- I'm the King of the Photobomb!

Some scraggly-looking guy ruined a perfectly good selfie of 50 Cent and Naomi Campbell ... oh wait, that's just Leonardo DiCaprio.

50 posted the epic photo over the weekend, writing, "I get photobombed by the hottest people in the world."

Leo's been rocking the homeless look for some time now ... but it didn't stop him from ringing in the New Year on a private island in St. Barts with a bunch of hot chicks.