The unidentified concertgoer at London's Hard Rock Calling told People magazine the former "Friends" star was with the singer after the show.
"She was on the side of the stage and really couldn't be seen by anyone," the onlooker said. "John didn't make any call-outs to her during the Hard Rock Calling show, but she was seen hanging out with him in the Artist Area before and after his performance."
Aniston was also spotted at Mayer's Friday performance at another London locale, People said.
The celebrity couple were also spotted together earlier in the week in the Netherlands, where Mayer was performing and Aniston was promoting her newest film "Marley and Me."