The soldier, Charles Jeffries, sent the card from Limburg an der Lahn on April 30, 1918 to let his family in Westcliff-on-Sea, Essex, know he had been taken prisoner.
His 78-year-old granddaughter, Pat Nicholls of Shepreth, Cambridgeshire, had the card in a file of family memorabilia.
She is trying to find out more about the role her grandfather played in the war, 'BBC News' reported.
The card gives Jeffries' regiment as the Royal Naval Division but does not indicate whether he was wounded.
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Nicholls has had the postcard in a file of family memorabilia.
The card, which Nicholls has handed over to historians in Shepreth, is printed in a mixture of German and English and headed "I am a prisoner of war in Germany."
Nicholls said Jeffries died of lung cancer in 1953 while she was a teenager.
"I remember him well. I wish I'd asked him about the war. But people didn't talk about it," she said.
"He was a waterworks inspector and worked for the Southend Waterworks Company.