Cops were called to the Barbados house Sunday where Rihanna is staying -- presumably topless -- after a man trespassed onto the property.

Barbados Police tell TMZ ... they received a call from someone in Rihanna's party after a man believed to be a German national (above) wandered onto the private, beachfront estate.

By the time cops arrived, Mr. Beerfest had already beat a hasty retreat -- and we're told at no time was the singer in danger.