Cristiano Ronaldo -- It Was a Beard All Along!

Seems Cristiano Ronaldo has a lot more free time on his hands now that he's single -- 'cause the soccer star decided to go under cover as a fat guy to screw with some fans.

Ronaldo threw on a beard, mustache, sunglasses and some fat guy padding ... and went out on the streets of Madrid Thursday to juggle a soccer ball ... just to see how people would react.

Eventually, Ronaldo ripped off the disguise ... and the people went crazy ... especially one young kid who got a hug from Ronaldo and an autographed soccer ball.

He's so sneaky.