A fisherman pulled the beer bottle with the scribbled message out of the Baltic off the northern city of Kiel last month.
It contained a postcard dated May 17th, 1913 written by a man named Richard Platz.
The modest Danish postcard had two German stamps on it and a polite message asking the finder to send it on to his address in Berlin.
Holger von Neuhoff of the International Maritime Museum in Hamburg said: "This is certainly the first time such an old message in a bottle was found, particularly with the bottle intact."
Researchers then set to work identifying the author and managed to track down his 62-year-old granddaughter Angela Erdmann, who lives in Berlin.
"It was almost unbelievable," Erdmann told news agency DPA. She was first able to hold the brown bottle last week at the Hamburg museum.
Inside was a message on a postcard requesting the finder to return it to his home address in Berlin. "That was a pretty moving moment," Erdmann said. "Tears rolled down my cheeks."
Von Neuhoff said researchers were able to determine based on the address that it was 20-year-old baker's son Richard Platz who threw the bottle in the Baltic while on a hike with a nature appreciation group in 1913.
A Berlin-based genealogical researcher then located Erdmann, who never knew Platz, her mother's father who died in 1946 at the age of 54.
Von Neuhoff said a handwriting comparison with letters penned by Platz later in life confirmed that he was "without a doubt" the author.
Erdmann told local newspapers that the surprise discovery had inspired her to look through family scrapbooks to learn more about her grandfather, a Social Democrat who liked to read.
Much of the ink on the postcard has been rendered illegible with time and dampness, von Neuhoff said.
The discovery will be on display at the museum until May 1st, after which experts will set to work trying to decipher the rest of the message.
The Guinness World Records had previously identified the oldest message in a bottle as dating from 1914. It spent nearly 98 years at sea before being fished from the water.