With Perry donning a printed bikini and Mayer sporting a casual tee, the pair was photographed at a pool party in Beverly Hills August 4, openly kissing and cuddling, reported Us magazine.
"They hugged each other and splashed around. They were all over each other, making out! Neither of them is exactly shy," said an onlooker.
Perry, 27, is diving headfirst into a relationship with Mayer, 34, according to multiple sources.
"Katy says she's officially dating John, not just hooking up," says one insider. Another source says the "Teenage Dream" singer "doesn't want to get her heart broken again, but she feels a real connection and wants to see where things go."
After brief flings with Baptiste Giabiconi, a hunky male model, and Robert Ackroyd, a guitarist for Florence + the Machine, Perry was ready for something more serious -- and Mayer was looking for the same.
The "Shadow Days" singer, whose ex-flames include Jennifer Love Hewitt, Jessica Simpson and Jennifer Aniston, hadn't been in a serious relationship since 2009.
But in a recent interview, the singer/songwriter insisted he's a changed man: "I haven't had a girlfriend in a long time... Now I feel like I'm extremely open to love."