Europe's longest-serving monarch, Harald is suffering from haemolytic anaemia, according to the palace - a condition caused by an abnormally rapid destruction of red blood cells which leads to fatigue, paleness, and shortness of breath.
"The king has been treated with cortisone in recent weeks" to combat the illness, the palace said in a statement.
"This treatment has led to fluid retention in the body, which requires hospital care."
His son, Crown Prince Haakon, will serve as regent during his sick leave.
Harald, who ascended the Norwegian throne in 1991, has had multiple health problems in recent years, forcing him to have a pacemaker fitted and to scale back his official schedule.
But he has always ruled out abdicating, arguing that he took a lifelong oath before the Norwegian Parliament.
In February, he was hospitalised for an infection and dehydration in Tenerife, where he was on a private trip with his wife, Queen Sonja.
The queen, also 89, was briefly admitted to hospital in May due to heart problems.
The couple's health problems come after a torrid period for the Norwegian royals, notably in connection with Crown Princess Mette-Marit, who is married to Haakon.
Documents published in January in the United States revealed her sustained and at times intimate correspondence with sex offender Jeffrey Epstein between 2011 and 2014, at a time when the American financier had already been convicted.
The princess has also had to contend with the legal troubles of her son, Marius Borg Høiby .
Born from a relationship prior to Mette-Marit and Haakon's marriage in 2001, Høiby , 29, was sentenced in June to four years in prison for two rapes and 32 other charges, including violence against a former partner. He has appealed the verdict.
Suffering herself from an incurable lung disease, Mette-Marit, 52, also had to undergo a lung transplant in July.
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