The artists are "very serious" about each other, a source tells E! News, and we're hearing that those close to the couple wouldn't be surprised if they got engaged in the not-so-distant future.
We know that Perry thinks the world of her beau, calling him a "genius" in an interview with Billboard last month and dishing on his bedtime crossword puzzle habit.
"I've been a fan of John's for years before I got to actually kiss him," she also told E! News at the MTV Video Music Awards. "But I've been such a fan, I think he's so incredibly talented and gifted and a genius songwriter. So to be a part of anything he was doing was just natural." Perry and Mayer duet on the tune "Who You Love," off of his Paradise Valley album.
The duo have also been spending tons of time together, enjoying a day at Disneyland earlier this month and then hanging out in New York this past weekend while Perry was in town to perform on Saturday Night Live.
And, according to the New York Post's Page Six, they also hit the dance floor at Mayer's brother Carl's wedding on Sunday at the Mamaroneck Beach and Yacht Club in Westchester County.
"They were very polite and down-to-earth and didn't want to take away from the bride and groom's day," a source told the paper. "Katy stayed behind the scenes and politely turned down photos. It was a beautiful ceremony on the water."
Reps for each have not yet responded to requests for comment.
Mayer, who turns 36 tomorrow, had to head right back to work after the nuptials—he's got a sold-out show in Herning, Denmark, tonight and then will spend his birthday in Frederiksberg, where he'll perform at the Forum Copenhagen.