The 'Talk That Talk' wasn't supposed to be singing at the Radio 1 Hackney Weekend festival until tonight, when she's headlining herself.
But she surprised crowds at Hackney Marshes by hitting the stage when American star Jay-Z was playing.
Rihanna wowed the crowds dressed all in black - wearing cut-off shorts, a crop-top and a sheer cardi.
She completed her look with a pair of futuristic John Lennon-style glasses and a leather head piece.
Rihanna accompanied Jay-Z on stage for his first track 'Run This Town' as the crowd went crazy despite the rainy weather.
And Ri Ri wasn't the only superstar that Jay-Z welcomed onto his stage. US star M.I.A also joined to sing 'Paper Planes' before her mic failed and the two duetted 'Bad Girls'.
[Related story: Rita Ora does PJ chic at Hackney Weekend]
[Related story: Leona Lewis channels Cheryl Cole's prints at Hackney]
And fellow rapper Kanye West also hit the stage with him to raise noise levels higher.
Earlier in the day Rita Ora, Dappy, Emeli Sande, Leona Lewis, DJ Fresh, Rizzle Kicks and a whole load of other acts hit the Radio 1 Hackney Weekend stage.
And X Factor songstress Leona wowed crowds as she channelled Cheryl Cole's print action.