The Fast & Furious star, who tragically passed away in a car accident on 30 November, happened to be in a Santa Barbara, California jewellery shop when he overheard a couple talking about their dream engagement ring.
Iraq war veteran Kyle Upham and his new wife Kristen got chatting to the star and at the time suspected he had paid for the £5,500 ring, but were only told by the jewellery store now.
"It's still to this day the most generous thing anyone has ever done for me," Kristen told CBS. "I noticed there was someone else in there but didn't give it much thought, and we started looking at rings and whatnot and he kept saying, 'Go bigger' and I kept saying, 'No, look at the prices.'"
Kyle then started chatting to the fellow shopper, who turned out to be Paul Walker, and explained that he had just returned from his tour of duty.
"When he found out Kyle just came back from Iraq, I remember seeing the look in his face," said Kristen. "He kind of transformed."
The couple left the store without the diamond ring, but shortly after received a call from the jewellers asking them to come back.
"One of the ladies came out holding a bag and just said, 'Here's your ring,'" said Kristen. "I think both of our mouths dropped."
When Kyle and Kristen asked who had paid for the ring, the jewellers refused to say, at Paul's request. The store had kept the 40-year-old star's act of kindness a secret until now, when employee Irene King felt the world should know about Paul's charitable deed.
Speaking about his late son, Paul Walker Sr explained how he was coping with the family's loss.
"It's hard. Every now and then I'll really break down," said Paul Sr. "Talking really seems to help. Remembrances, instances, talking about him.
"There are tremendous amounts of stories, like I was just told that my son gave a marine a diamond ring to give to this girl he was going to marry. I never heard that story. It's stuff like that, all the time. I think he liked people not knowing."
On the day of his death, Paul, who founded charity Reach Out Worldwide, was attending a fundraiser that his organisation had put on.
The father-of-one and his friend Roger Rodas, who was driving the car, took the Porsche out for a spin when they crashed into a lamppost and tree. Both men tragically died at the scene.