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.
Here's what those ticket holders missed.