The singer recently wrapped up the European leg of her Prismatic World Tour in London. She threw a party to celebrate at the O2 Arena, where she was seen cosying up to her former flame.
"Katy and Rob seemed to be very much back together at her party on Saturday night," a source told the Sun.
"They were inseparable, kissing, flirting and cuddling away in front of the roadies, celebs and friends at the bash.
"There were a fair few famous faces in the room but Katy and Rob only had eyes for each other."
Katy, 29, and Rob briefly dated back in 2012, following her split from husband Russell Brand. The Roar singer broke up with boyfriend John Mayer in February after nearly two years of dating, and recently ended her month-long romance with American DJ Thomas Pentz, aka Diplo.
Speaking to Chelsea Handler last month about the end of her relationship with John, Katy said, "I know it sounds really hippie-s**t, but I'm living in the now and not fantasising about the future of (being) torn up about the past.
"…I have a lot of respect for John still, and I know it goes both ways. There's no bad blood, but I'm sure there will be inspired songs."