The Firework hitmaker took her California Dreams tour to the Nokia Theater on Friday, playing her hits to a sold-out crowd of more than 7,000.
But one of the most surprising moments of the night came when Katy, 26, started singing Black's hit tune Friday, before bringing out the 14-year-old to join her for the chorus.
Rebecca first shot to fame earlier this year, when her track Friday was a big hit on YouTube.com.
She then went on to feature in Perry's video for her latest single Last Friday Night [T.G.I.F.].