The film -- an almost complete version of "Empty Socks", the first Christmas film by the US entertainment giant -- was discovered during an inventory at the library's facility in Mo i Rana, near the Arctic circle.
"At the beginning, we didn't know it was a lost cinematographic treasure," Kvale Soerenssen, an archivist at the library, said in a statement.
"The film was in two reels which weren't clearly labelled."
The original version of "Empty Socks" had a duration of 5:30 minutes, but between 30 and 60 seconds from the central part were missing in the footage found in Norway, which was authenticated by Disney cartoonist David Gerstein.
The film stars Oswald, the Lucky Rabbit, a predecessor of Mickey Mouse who appeared in 26 movies by Walt Disney and his countryman Ub Iwerks.
The National Library said it had digitized it and sent a copy to The Walt Disney Company.
Before this find, the only known copy of "Empty Socks" was a 25-second sequence preserved at the Museum of Modern Art in New York.
The Norwegian film originally belonged to a private individual, before ending up in the collection of the Norwegian Film Institute, which handed over its archive to the National Library of Norway.
Most of the library's works and documents are kept in a high-security bunker in Mo i Rana.
According to Norwegian newspaper Aftenposten, the nitrocellulose reels -- a highly flammable compound -- are stored in a room with controlled temperature and humidity.
The National Library has launched a large digitisation project to make its collection more accessible, which has helped discover lost documents.