Los Angeles - Katy Perry originally dated John Mayer before she met Russell Brand.
The Part of Me singer reportedly went on a date with the guitarist - who she is now in a relationship with - the day before she met the comedian at the MTV Video Music Awards (VMAs) in Los Angeles, which eventually led to their marriage in October 2010.
A source told LOOK magazine: "A couple of friends had set Katy and John up on this date. They had an amazing time and were planning to do it again.
"Then she went to the VMAs and fell head over heels for Russell. But it must have always felt like unfinished business with her and John, and they'd occasionally text each other while she was with Russell. It's no surprise to friends they have hooked up now her divorce has been finalised."
Russell and Katy split in December 2011 after 14 months of marriage and their divorce was finalised a couple of weeks ago. She had been dating Florence + the Machine guitarist Rob Ackroyd, but insisted it was not serious between them.
Friends say that John - who has previously dated a string of beauties including Taylor Swift, Jessica Simpson, Cameron Diaz and Jennifer Aniston - is even prepared to put his promiscuous past behind him and settle down with Katie.
A source added: "He told her right from the start that he was going to 'cure' her heartache over Russell and wanted to meet 'The One', so he could settle down and marry. He was clearly on a mission to get her to fall for him and it worked."
Katy is already said to be smitten with her new man, falling "hook, line and sinker" for him.
The source added: "She's considering giving him a key to her Hollywood apartment. She says it's 'just in case she gets locked out,' but we know there's more to it than that."