And things are going so well between the two lovebirds that Mr. Brand is already bonding with Perry's parents.
"While the crowd was singing, 'Happy Birthday,' Russell had his arm around Katie's dad's shoulder," a partygoer says.
Another source said Perry gushed to a pal about her new beau: "Do you want to meet my boyfriend? He's hilarious!"
As for the festivities...
Perry started a food fight with her birthday cake. "There was cake all over the floor, and the crowd started dancing on top of it to 'Like a Virgin,' a source said."
There was also paint-splattering. "Katy was covered in paint," one of our sources reported.
Folks enjoyed not only a smorgasbord of candy and cake but also drinks like 42 Below vodka's custom-made Golden Ticket and Everlasting Gobstopper cocktails.
The good times continued later on at Hollywood nightclub H.Wood, where Lindsay Lohan popped by.
"Lindsay walked into the bathroom with a few girlfriends, complaining that some guy scratched her cornea," says a source. "Lindsay's friends kept telling her to find some contact solution and flush out her eye. She left really quickly."
Which leaves the party girl crown up for Perry's grab. She and Brand were also spotted on Friday having dinner with friends at Nobu in West Hollywood.
—Reporting by Dahvi Shira and Whitney English
What else went down this weekend? Take a look at our Big Picture photo gallery to find out.