Konrad Fischer, captain of a fishing boat from Schleswig-Holstein, told Kieler Nachrichten, a local newspaper, that he was about to throw the bottle back in the sea when someone told him it was not empty, the Local.de reported.
"When I saw the date I got really excited," he said.
The postcard, from Denmark, had German stamps on it. It was signed by Richard Platz and addressed to his own home in Berlin with a message asking the finder to put it in the mail.
The Guinness Book of World Records lists the oldest previous message in a bottle as 97 years, found in 2012.
Fischer said his previous finds during his 50 years as a fisherman have been more alarming, including mines, bombs, torpedoes and a body.