A California man who was killed in a car accident is being honoured by a woman he helped at a grocery store.

Matthew Jackson, 28, was killed in a car accident on Nov. 11 less than 24 hours after helping distressed Jamie-Lynne Knighten pay for her groceries at a Trader Joe's store.

Now Knighten wants the world to know about Jackson's act of kindness.

She said on Nov. 10, she went to Trader Joe's with her baby Wyatt as she needed to get groceries.

"It was by far the most stressful shopping experience because he was also out of his routine and fighting sleep," she wrote on Facebook.

At the check-out counter, Wyatt was crying and Knighten realised that she had left her debit card on the kitchen counter. She tried using her credit card but it got declined several times, realising that her bank put an anti-fraud hold on it when she traveled to California.

It was then that Jackson, who was behind her, said, "May I? May I take care of your groceries?"

She said, "Oh, thank you ... How sweet, but no thank you, you don't have to do that."

He repeated his offer and said, "I would be glad to take care of your groceries as long as you promise to do it for someone else."

Knighten accepted his offer and Jackson paid for her $200 groceries.

"Thanking him endlessly through my tears I ask his name and where he works before parting ways," she said.

She tried calling him a week later and was stunned to find out that Jackson was killed in a car accident in Oceanside on Nov. 11.

"Not even 24 hours after meeting him ... Gone. Just like that. How? Why? I don't understand. He was a year younger than me and engaged to be married. His boss explained to me how amazing this young man was in his everyday life and that what he did for me was just who he was as a person," she wrote.

She added, "I still cannot believe it. I thought for sure I would get the chance to see him again, give him a hug and thank him at least once more in person. Now I won't get that chance, but more importantly no one else will get the chance to meet him. And that breaks my heart."

Knighten launched the website www.matthewslegacy.org urging people to do something extraordinary for a stranger as a tribute to Jackson.

Jackson's mother, LeeAnn Krymow, knew that his son was a good boy.

"I knew my boy was like this," Krymow told the San Diego Union-Tribune. "He loved to be kind. He was just a really special kid. So cute, so intelligent, so talented, an accomplished musician. You wonder why these things happen."

She added that she is glad Jackson will be remembered for his good deeds.

"There has got to be some good to come of this," she said. "He would be happy to know that other people are learning from his example."