The actor arrived on stage for the surprise duet – joining Bruno and his musicians in a rendition of James Brown's hit Sex Machine.
He danced along to the music and followed the lead of Bruno's professional percussionist at a fundraising gala for Brad's Make It Right foundation in New Orleans.
Brad Pitt joined Bruno Mars on stage, during a charity gig, to play the tamborine
Brad helped devise the charity evening, which raised housing funds for those affected by Hurricane Katrina, as well as other people in need around the US.
He is said to have told the crowd after his turn on stage: "I love New Orleans...And I'm extremely proud of the Lower 9th Ward community."
The star was referring to the a community-driven, nonprofit organisation and neighborhood centre based in the hurricane-hit city's Lower 9th Ward.
They have been commended for their work in helping residents get back on their feet and revitalising the community.
Brad was joined at the event by his partner Angelina Jolie, and a host of other stars who contributed by paying between $10,000 to $25,000 a table.
Individual tickets, which sold out well in advance of the event, went for $1,000 a piece. Those in attendance included Modern Family star Sofia Vegara, comedian Chris Rock – who was on hosting duty - and Sandra Bullock, who spoke at the event.