The 24-year-old singer was near the pool in her native house of Barbadose, where she is currently on a holiday, when the stranger walked inside.
"The guy got to within about 10 to 12ft of where Rihanna was sitting before he was apprehended by security. He looked fairly harmless but it was a little too close for comfort," thesun.co.uk quoted a source as saying.
The man is said to have travelled to the Caribbean island specifically to try to meet the star. Wearing just shorts he had reached the 10-bedroom villa by swimming on to a beach at the back.
The man was arrested, quizzed and discharged later.