The 'Wide Awake' - who divorced British comedian Russell Brand in December 2011 after 14 months of marriage - was spotted being very affectionate with the 'Half Of My Heart' singer at a joint birthday bash for 'Hello Giggles' blogger Sophia Rossi and 'New Girl' co-creator Liz Meriwether on Saturday night (10.11.12) and insiders say she is "obsessed" with him now they are "full on dating".
A fellow party guest told US Magazine.com: "John had his hand on Katy's back a lot, and they were kissing and holding hands."
The couple's date at the party - which was also attended by A-listers Jon Hamm, Mindy Kaling, Nicole Richie, Shannon Woodward and Zooey Deschanel - is just the latest in a string of romantic high profile outings for the duo since they rekindled their romance in September following a brief split in August.
A source previously revealed that they barely acknowledged other guests at Katy's 28th birthday party last month because "they were in a world of their own."
An insider said: "She didn't even talk to her friends much. It was just her and John. She's obsessed with him. She said hello to people but then went right back to him."
Friends say Katy "believes she can change" John and "doesn't want to be just another one of his conquests."
The 35-year-old rocker has previously romanced Taylor Swift, Jessica Simpson, Jennifer Aniston and Jennifer Love-Hewitt.