"'Surprise guest at Staples Center: @katyperry!!" Taylor posted on her Twitter page after the show. "We sang Hot N Cold and jumped up and down a lot. Katy! LA! I will always love you!"
Outfitted in a black sleeveless dress and black boots, Taylor was just getting into the song, dancing toward the audience and handing one lucky fan in the front row a bouquet of flowers, when a roar erupted in the audience.
Turning, Taylor announced, "Ladies and gentleman, please welcome to the stage Katy Perry!"
The pop singer -- dressed in a blue shirt, short white skirt, white boots, and black tights, ran down the steps from a riser and joined Taylor center stage.
The two jumped up and down like high-school cheerleaders on the sidelines at a big game as they sang Katy's huge pop hit together. At one point, Taylor yelled, "Let me see you jump," and many in the screaming crowd started to bounce along with the two beauties.
Katy did give fans a hint of what was to come earlier in the day, when she sent an all-caps tweet to Taylor: "@TaylorSwift13 CAN'T. WAIT. FOR. DANCE. PARTY."