The rapper drew huge cheers from the crowd in Chelmsford, Essex, when the 'Rude Boy' singer - who also performed a set earlier in the evening - joined him for their collaborative single 'Love The Way You Lie'.
An onlooker told BANG Showbiz: "Everyone was hoping Rihanna would join him up there, but having been on stage directly before Eminem we all thought she'd be too wiped out to sing anymore.
"But they were absolutely amazing together and the audience went wild!"
After performing hits including 'My Name Is' and 'The Real Slim Shady' Eminem closed his set with 'Not Afraid', before an encore of 'Lose Yourself'.
Jessie J - who appeared on stage with her foot still in a cast after she broke it earlier this year while rehearsing for a concert in London - told fans she appeared because she had vowed to herself to perform after attending as a fan last year.
She told the crowd: "Last year I was here in the crowd watching. I saw Florence and the Machine and said to myself, 'Next year I'm going to be there on the stage'.
"Thank you to everyone for making that happen."
There's half of a great satire here, as Seth Rogen, James Franco and Evan Goldberg...
This lively and engaging documentary may be set out like an informercial, but it teaches...
Hip hop has always been more of a culture than just a genre of music....