It happened late last month at the Bull's Head, a riverside pub in southwest London, and anyone who wants proof can see grainy footage on YouTube that someone captured on a cell phone.
Mick Jagger, 63, had been visiting his former wife Jerry and their children in nearby Richmond when he slipped into the pub wearing a purple sweater and joined Chris's Atcha! blues band.
In the impromptu performance, Chris, 59, plays guitar, and the brothers use a single microphone to sing "Dead Flowers," a track from the Stones' "Sticky Fingers" album.
Chris told the Daily Mail newspaper that he persuaded his brother during a break in the Atcha! show to take to the stage.
"At first he didn't really want to do it, but I said to him: 'You've got a tour coming up. I think you need the practice,'" Chris said.
The Rolling Stones began a European tour in Belgium this week and will be headlining at the Isle of Wight music festival on Sunday.