The "Teenage Dream" singer and her BFF were spotted letting loose Thursday night at West Hollywood hotspot Voyeur.
"They were dancing and drinking, having a blast with a group of friends," a source tells us of the chart-topping gal pals. "The energy climaxed as they were all were jumping up and down when DJ Devin Lucien played Katy's hit single 'Firework.'"
Earlier that day, Ri-Ri taped a performance at American Idol and grabbed dinner with friends at L.A.'s Osteria Mozza restaurant.
But wait. That's not the only celeb sighting we have for you. Read on to find out where newly single Hollywood hottie Zac Efron was spotted...
DINING: Efron and five pals stopped by Fig & Olive in West Hoolywood for dinner, sampling orders of lamb chops, sirloin steak and four different desserts....Kim Kardashian and beau Kris Humphries made it date night plus one when they were joined by Jonathan Cheban at Beaty & Essex in NYC....Scream 4 starlet Hayden Panetierre grabbed sushi and skewers with a group of guy friends at Sunset Strip eatery Katana in Hollywood....Adrien Grenier hit Napa Valley Grille restaurant in Los Angeles with five friends.
VACAY: Audrina Patridge took some vacation time with pals in Cozumel, hitting the beach for boat rides and snorkeling.
PARTYING: Emma Stone and James Marsden sipped Bombay Sapphire cocktails while mingling with the likes of Vanessa Hudgens, Sofía Vergara and Brooklyn Decker at the Conde Nast Traveler Hot List party at the Soho House in West Hollywood... Dancing dude Maksim Chmerkovskiy was spotted drinking Corzo tequila while hanging with friends at Playhouse Hollywood nightclub.
DOWN TIME: Thor's Chris Hemsworth and wife Elsa Pataky shopped at L.A.'s G-Star Melrose location... Leona Lewis stopped by the Nail Garden in L.A.'s Studio City neighborhood with a girlfriend to enjoy some mani/pedi pampering.
—Reporting by Whitney English