The former couple have dated off and on since last summer, confirming their second breakup in March.
However, Mayer was a guest at Perry's Memorial Day barbecue in Hollywood Hills, the magazine said.
"They seem to be back together -- or back together-ish," one source said. "They were very flirty and having a really good time together. They have good chemistry."
"They were definitely friendly," a second insider said.
2014: The Year in Music [PHOTOS]