"It was lying in the middle of the pile of post and I thought that it was so cool when I saw it," said Agneta Hartberg at the surgery to the newspaper.
The envelope was post-marked August 15th 1936 and had been sent with the "Svenska Amerika Linien", which was a Swedish shipping line which plied the route from Gothenburg to North America at the time.
While the sender of the letter remains a mystery as the envelope remains intact, the search has begun for the intended recipient, a "Mister Arne Löwgren".
The local post office has meanwhile expressed surprise over the emergence of the letter after all these years.
"I can say that this is not a normal standard of service at the post office. It doesn't normally take this long to deliver a letter. It is actually very exciting," said Gotland post office head Ronny Larsson to the newspaper.
The unusual letter is post-marked Stockholm and appears to have stamps in Russian and writing in French on the back. It also bears the stamp of the M/S Gripsholm which reads "New York-Viking Lands-Russia-New York".
The M/S Gripsholm was a passenger liner which was delivered to the Svenska Amerika Linien in 1925.
While the search continues for any surviving relatives of Arne Löwgren, the staff at the dentist surgery are doing their best to resist the temptation to open the letter and reveal its contents.
"Of course we are itching to find out," said Agneta Hartberg.