Europe's oldest reigning monarch died peacefully at 6.35am on Friday at Oslo University Hospital, the royal house said.
Harald had been in hospital since August 17, receiving treatment for haemolytic anaemia, which had led to fluid retention in his body.
He subsequently contracted a bacterial infection of the bloodstream.
The Norwegian king had long been in poor health.
In February, the monarch had been taken to hospital with an infection while on holiday in Tenerife with his wife, Queen Sonja.
Walking had also become difficult for him in recent times.
Despite this, Harald had ruled out stepping down from the throne early in favour of Haakon.
Norwegians valued King Harald V for his reserved manner, and for his ability to take a clear stand when it mattered.
He slowly introduced modern touches that brought a ceremonial post seen mainly as a symbol of national independence into the 21st century.
A great-great-grandchild of Britain's Queen Victoria, Harald was King Charles III's second cousin once removed.
The UK monarch on Friday praised his "dear cousin" for his "unwavering dedication, guiding Norway through times of great change with warmth and humility".
The popular Harald played a crucial role comforting the nation in times of mourning.
When far-right, anti-Islam fanatic Anders Behring Breivik killed 77 people in 2011 in Oslo and on Utoeya island, the king consoled the nation with a powerful televised speech, saying "freedom is stronger than fear".
Following a refugee crisis in 2016, he positioned himself as a measured voice for diversity and tolerance.
The monarch was also popular for his dry humour.
He was an enthusiastic sailor and winter sports enthusiast, competing in several Olympics but failing to repeat his father's feat of winning Olympic gold.
Harald acceded to the throne in 1991 following the death of his father, Olav V.
He married his school sweetheart Sonja in 1968, even though his father had long struggled with his son's relationship with a commoner.
Harald was more tolerant with his own children: his son Crown Prince Haakon also married an ordinary Norwegian woman, Mette-Marit.
His daughter Martha Louise is now married for the second time to self-proclaimed shaman Durek Verrett.
Without Harald, the future direction of the Norwegian monarchy is uncertain.
While the king enjoyed enormous support in his country, scandals have recently shaken confidence in the crown prince's family considerably.
Crown Princess Mette-Marit has faced criticism over her close friendship with convicted US sex offender Jeffrey Epstein.
Following the king's death, constitutional lawyers said the 53-year-old would not automatically become queen - the decision on her role rests with the new King Haakon.
Mette-Marit's eldest son Marius Borg Hoiby, from a previous relationship, was sentenced in June to four years in prison under Norwegian law, among other things for two counts of rape.
The verdict has not yet become final.
Many Norwegians have since been uncertain whether Mette-Marit truly has what it takes to be queen.
with Reuters and PA
17°C