Rihanna Shocks Oldchella Crowd & Performs With Paul McCartney

Rihanna surprised everyone at Paul McCartney's desert show Saturday night ... joining him onstage for their hit song 'FourFiveSeconds.'

Paul was in the middle of his set during weekend 2 of the Desert Trip Festival outside of L.A. when RiRi entered stage right ... giving "Oldchella" a youthful Coachella-esque jolt.

"FourFiveSeconds" was co-written by McCartney and Kanye ... but Ye didn't make the trip for this jam. Paul performed the song last weekend, but minus Rihanna.