John Mayer surprised a fan by purchasing her dream guitar for her.

The 'Free Fallin'' singer and girlfriend Katy Perry popped into a guitar shop in Soho, New York, last Friday (05.07.13) and after spotting 16-year-old Julie Fermin admiring the expensive instrument John decided to buy it for her.

John asked the teen - who has been playing guitar since she was 11 - which instrument she liked best and then took it to the till.

She told Us Weekly magazine: ''We started freaking out and getting super excited.''

She added: ''I showed him, and he went over and checked the guitar's price, I said, 'Yup, but I can't afford it,' and he said, 'Oh, stop it!''

Both stars posed for pictures with the fan -which she later posted on Twitter - and said their goodbyes, when an employee revealed to the fan John had bought the guitar as a gift for her.

She gushed: ''I was so shocked, I started crying and screaming. I cannot describe all the emotions I felt at that moment. I just knew I was the luckiest and happiest person alive.''

The lucky fan took to Twitter to share her excitement.

She wrote: ''I never in my life thought I'd meet and get to talk to Katy Perry and John Mayer, I literally thought it was impossible.FOREVER MIND BLOWN. (sic)''