Phyllis Ponting, from Wiltshire, assumed her then-fiance Bill Walker had died in the conflict and gave up hope of hearing back from her letter accepting his marriage proposal.
But incredibly, nearly 80 years later, his reply was found at the bottom of the sea.
The letter was salvaged from the wreck of the SS Gairsoppa, a steam-powered cargo ship 300 miles off Ireland's coast.
A single torpedo from a German U-boat sank the cargo ship, around 300 miles off the coast of Galway in February 1941.
Eighty-three of the 84 crew members died in the shipwreck.
Ms Ponting, who was reunited with the letter by the BBC's The One Show, told the programme: "I can't believe the letter was at the bottom of the sea and now I can read it.
"I don't think Bill can have survived the war, otherwise he would have been straight round to my address in Roseland Avenue.
"We would have been married. He loved me a lot."
In the letter, Mr Walker spoke about her acceptance of his marriage proposal and wrote back: "I wish you could have been there when I opened it.
"I wept with joy. I could not help it. If you could only know how happy it made me, darling."
Mr Walker's letter was one of the 700 to survive in the wreck.
They were found by marine archaeologists three miles below the waves.
The ship was carrying 48 tonnes of silver when it was sunk.
Around 1,200 silver bars worth about £23.7 million were also recovered.
American-backed salvage firm Odyssey Marine Exploration says the rescued haul is the heaviest and deepest recovery of precious metals from a shipwreck ever made.
The company, under contract to the British Government, will get to keep 80% of the haul after expenses. The remaining 20% will go to the Treasury.
The letters are now part of a heart-rending exhibition called Voices from the Deep, being staged at London's Postal Museum.
She has since been visited by London Postal Museum representatives and given a copy to keep.
Museum curator Shaun Kingsley said: "It's the largest collection of letters since people started to write to survive any shipwreck, anywhere in the world.
"It shouldn't have been preserved, but because there was no light, there was no oxygen, it was darkness, it was like putting a collection of organics in a tin can, sealing it up and putting it in a fridge freezer.
"And in the conservation lab, slowly and suddenly words started to appear. Some 700 letters written from British India in 1940."
Ms Ponting went on to marry another man who she had four children with. She now has four grandchildren and seven great-grandchildren.