Always the fashion plate, as per The Hollywood Reporter, the glam singer wore a dress that resembled a giant ballot.
She warmed up the crowd for the free gathering in a park and performed "Let's Stay Together," "Part of Me," "Teenage Dream," "Wide Awake" and "Firework."
Katy's adorable 91 year-old grandmother was there and gave the president a smooch on the cheek! (Yes, it made a lipstick mark on his face).
The POTUS joked he had to wipe it off quickly before First Lady Michelle Obama would see it.
One insider dished, "He told the crowd about being in the midst of a marathon tour and ticked off all the stops. He said he was pulling an all-nighter, adding that if you're not going to sleep, 'you might as well be in Vegas.'"