Danny Cooper found the keys in the door to the side of the pillarbox on Cemetery Road, Pudsey, Leeds. He told the YEP it was not the first time the keys had been left.
Mr Cooper, 32, locked up the compartment and took the keys home with him before returning them to Royal Mail.
He claimed that only two weeks earlier his next door neighbour had to shout to a postman to warn him he had left his keys in the door of the post box on Westdale Drive and Standale Avenue.
He said: "Identity theft is big business and at the moment every penny counts. The last thing people want are black marks against their name through no fault of their own.
"I was just going home with my girlfriend Zoe when we saw the box on the side of the letterbox open and we didn't know what to do at first.
"It's a criminal offence to mess with the post so we just locked the door and took the keys home to stop anyone from going through it.
"Anyone could have got hold of the keys and who knows what could have happened, especially when you hear so much on the news about identity theft.
"All it takes is for someone to get hold of your address and postcode then who knows what could happen.
"I know people that this has happened to before because someone had registered a mobile phone at their address and it took a lot of hassle to sort out.
"People need to be aware of this as someone could have quite easily stolen another person's identity. All they would have to have done was to empty the mail boxes."
A Royal Mail spokesman said: "We take the security of mail incredibly seriously. It is one of our top priorities and we will be investigating."