The on/off couple looked cosy as they left the luxury Waldorf Astoria hotel together, despite Katy only recently saying that it was definitely "over" between the two.
Katy, who split from ex-husband Russell Brand in December 2011, was seen with singer-songwriter John at an event to honour comedian Don Rickles at the Friars Club. It is believed that the pair arrived separately and Katy secretly joined him inside.
Inside the event, Katy, who stunned in a flowing peach silk gown, posed for a photo alongside a smartly-dressed John and American singer Natalie Cole, daughter of the late Nat King Cole.
Natalie tweeted: "Lovely evening with @johnmayer and @katyperry at the @friarsclub."
It has also been reported that the couple enjoyed a low-key date night at Greenwich Hotel and Bowery Hotel in New York City on Saturday (22 June).
Katy recently confessed to US Vogue that she was no longer with John but revealed, "I still am madly in love with him."
The 'Firework' singer was last rumoured to be back with the notorious ladies man earlier this month after he was seen leaving her house and heading to a recording studio.
Katy and John, who began dating summer last year, were first spotted on a romantic date night at the Chateau Marmont in California.