Rihanna joins Jay

RIHANNA joined Jay-Z and Justin Timberlake on stage for a surprise performance at London's Yahoo! Wireless Festival on Sunday.

The Pour It Up singer shocked fans when she appeared as Jay-Z rapped their collaboration track Run This Town.

Dressed in a leather dungaree-style skirt and white crop top Rihanna caused a frenzy from the 60,000 strong crowd when she walked out to join Beyonce's hubby and Justin.

The appearance topped off the already star-studded line-up for Yahoo! Wireless Festival's three-day bash held at the Olympic Park in Stratford, London.

Other big name acts appearing across the US-dominated weekend included Rita Ora, Emeli Sande, Kendrick Lamar, A$AP Rocky, Nas, Macklemore, Calvin Harris, Will.i.am, Snoop Dogg and Wretch 32.

Justin headlined the event on Friday night with Jay-Z taking over on Saturday before the pair came together for the finale under the Legends Of Summer title.

They performed some of their biggest hits as well as tracks from their new albums Magna Carta ... Holy Grail and The 20/20 Experience.

Following her performance Rihanna, 25, posed backstage at the event and posted a snap on her Instagram account.

She wrote: "#Wireless #LegendzOfTheSummer #Hov #JT #London you were on one tonight!! One Love!!"

And she showed off her new blinging grill in another snap as she posted: "I'm an outlaw bitch!!"

Keeping busy, Rihanna will now perform in Manchester tonight as part of her Diamonds World Tour after headlining Scotland's T In The Park on Saturday.