The Born to Run star played his second of two shows at Croke Park in Ireland on Sunday, and the over 80,000 fans in the stadium went wild when the U2 frontman joined Bruce on stage.
The two then sang a rendition of the American rocker's hit Because the Night, a song he wrote with another veteran musician, Patti Smith.
Bono wasn't the only U2 member in attendance - his bandmates The Edge and Larry Mullen were also on hand, but did not join their lead singer for the surprise performance.
Bruce first played Croke Park on Friday (27May16), and during his day off on Saturday (28May16), the Boss was spotted at a local bowling alley, while he also enjoyed a few pints at pubs in the area.
He continues his European leg of The River tour on Wednesday (01Jun16) in Glasgow, Scotland.