Keith, who's 56 and has worked at the store for five years, said: "I was just walking past the checkouts and saw a colleague talking to the lady who was a bit panicky and upset. I asked what the matter was and they said she'd tried to use the card twice already, but couldn't remember the number.
"I advised her not to try a third time as her card may have become blocked. She had no cash on her so I just said to her that I'd pay for her shopping. I think it was about £12. She was extremely grateful and I just said to her to enjoy her food and that I didn't want the money back.
"I believe in treating people how I would want to be treated. I like to make people happy when they are shopping with us."
Julie Lennox, the store's GM and George trading manager, said colleagues were proud of what Keith did and that he has been nominated for an Asda service superstar award for his kindness.
She said: "It really was excellent what Keith did; such a kind act. He is a very conscientious person and helpful at all times. He's very much a people person."
And the store's community champion Diana Morgan added: "You really were a knight in shining armour Keith. Thank you, what a lovely thing to do."