Lady Gaga joins U2 onstage at Madison Square Garden

U2's eight-night residency at New York's Madison Square Garden has had quite the celebrity guest list so far. Last night, Lady Gaga joined the rock band onstage to perform "Ordinary Love."

Lead singer Bono repeatedly said, "Goodness gracious," as Gaga climbed onto the stage from the audience, wearing a black bikini, fishnets and shoes by the late Alexander McQueen.

Bono introduced the "Born This Way" singer to the audience, saying, "Her Lady-ness, her Gaga-ness, is here this evening!"

Gaga hugged Bono before getting behind the piano, without saying a word. They performed the duet with Bono standing at the piano, opposite Gaga.

"Ordinary Love," a tribute to Nelson Mandela, was released in 2013 as a single for the film Mandela: Long Walk to Freedom.

U2's surprise celebrity guest so far has included everyone from Paul McCartney to Alicia Keys to Jimmy Fallon, according to Billboard.

Watch U2 and Lady Gaga's performance below.