Former Royal Engineer Leonard Wilkes, from Birmingham, central England, recorded neatly written entries into a small blue notebook during his time serving in France, the Birmingham Mail reported Tuesday.
His family claims it could contain the earliest written record of the story of Jean-Marie Loret -- who always claimed his father was Hitler.
Wilkes landed on the Normandy beaches on D-Day in June 1944, and a few months later recorded the following entry, "An interesting day today. Visited the house where Hitler stayed as a corporal in the last war, saw the woman who had a baby by him and she told us that the baby, a son, was now fighting in the French army against the Germans."
Wilkes never mentioned the woman's claim to his family, who discovered the notebook after his death in the 1980s.
The diary backs up a report in French magazine Le Point last month that claimed that while Hitler never acknowledged Loret as his son, he sent money to Charlotte Lobjoie, Loret's mother.
Wilkes' son Alan, 72, said, "I was shocked when I read it. My father never spoke to me about the war and, until he died, I never even knew the diary existed."