Rihanna -- Weirdo Arrested For Creeping On RiRi's Roof

Well this is bizarre -- a man was arrested last night for tiptoeing around Rihanna's roof ... but lucky for the singer, security guards nabbed the guy before he made it much further.

Law enforcement sources tell TMZ, private security guards near Rihanna's Pacific Palisades property called police just after 10 PM, reporting they had detained the 26-year-old alleged trespasser. Guards then held the man until officers arrived, and turned him over. The man was subsequently placed under arrest.

We're told the man was adamant ... he did NOT know it was Rihanna's house he was creeping around -- though cops tell us he had a Google map of the area pulled up on his phone. (Maybe he just got really lost???)

Bail was set $2,882. He's still in custody.

According to the L.A. Times -- which broke the story -- Rihanna wasn't home at the time ... she's in the U.K. on her Diamonds World Tour.

It's been a wild year for Rihanna's L.A. pad -- she was swatted in April ... and there was also that guy who tried to break into her house and sleep in her bed. The perks of being famous.