Cullum was left stunned when a man confused him with the Hawaiian superstar while waiting for a flight, but was too polite to reveal his real identity.
And the star ensured the fan had a memorable meeting with 'Mars' by bursting into one of his tracks.
Cullum tells Britain's The Sun, "Some guy at the airport came up to me and went, 'Hey, Bruno!' I was bit taken aback so I just went along with it and started singing Grenade.
"He was none the wiser, although he probably thought I was acting like an idiot."