A secret Santa has warmed the hearts of a whole community after the mystery man selflessly paid for every purchase at a US department store.

Walmart customer Julie Gates was picking up a package on Thursday when she overhead the anonymous man offering to pay for someone's Christmas presents.

"I don't even think if I can find the words to say," she told the outlet. "He came up to the man in front of me and said, 'Either I am going to pay for it now or I am going to pay for it later'," Gates told WCAX TV.

The man's generosity didn't end there, and he told Gates he would pay for her items too.

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"Why don't you run and get what you want now and come back here."

Items at a Walmart branch were paid off in full by one mystery shopper. Photo / Getty Images
Items at a Walmart branch were paid off in full by one mystery shopper. Photo / Getty Images

She was sceptical, but she did as he said and when she returned, he was still there.

The secret Santa had paid for everyone's items in the line.

Gates estimated that the people in front of her had up to US$900 worth of items.

But the good deed didn't end there.

On Saturday morning the store's layby room was filled to the roof with items for Christmas.

By Saturday evening, every layby item was paid for.

Walmart couldn't say how much the mystery shopper spent or how many people went home with free gifts.

One customer said money has been tight since he has been off work with a broken foot and the generous offer from the anonymous good Samaritan means they can get through Christmas.

"I really have a feeling that there is hope for mankind, there are still good people out there."