He was 69.
The Cappelen Damm publishing house did not specify the cause of death.
Ambjørnsen had long been public with his battle against a lung illness called chronic obstructive pulmonary disease, or COPD.
Norwegian tabloid VG reported he is survived by his wife, Gabriele Haefs, saying she posted news of Ambjørnsen's death on Saturday.
Born on May 20, 1956, in Tönsberg - described on his German-language author website as "Norway's most bar-filled town" - Ambjørnsen grew up in Larvik and worked in various jobs in industry and psychiatry before publishing his first documentary novel in 1981.
Four years later he moved to Hamburg, where he lived for decades.
"His books are characterised by powerful, realistic descriptions of the seamier side of life," the publisher said.
Ambjørnsen became one of the publisher's best-known contemporary authors with four novels built around the character Elling, a shy and imaginative outsider who coped with the funny but endearing foibles of daily life after release from a psychiatric hospital.
The comedy Elling - the story of two recently released mental patients bunking together in an apartment in Oslo - was nominated in 2001 for an Academy Award as best foreign-language film.
The tale landed on Broadway in 2010, with a play starring Denis O'Hare and Brendan Fraser: one of the misfits was fixated on his mother, the other obsessed with sex.
According to the author's website, Ambjørnsen wrote 18 novels and three collections of short stories as well as several books for children and youth.
A newly written collection of short stories is set to go on sale in Norway on July 31.
