The 83-year-old monarch, who does not have formal political powers, has been head of state since 1991.
The king first had heart surgery in 2005, and the operation scheduled for Friday will replace the artificial heart valve he received 15 years ago, the palace said.
“This time it will not be open heart surgery,” the palace said in a statement. “The king will be awake during the procedure.”
King Harald was hospitalised on Sept 25 due to a shortness of breath, although doctors quickly ruled out any Covid-19 infection. He was discharged three days later but has remained on sick leave as further tests were conducted, the palace said.
The king had undergone regular check-ups following his 2005 surgery, his personal physician Bjoern Bendz said.
“The latest results indicate that this operation is necessary to improve the king’s breathing,” Dr Bendz said in the statement.
The Oslo University Hospital carries out the procedure hundreds of times a year, he said. In 2003, King Harald also underwent surgery for bladder cancer at the same hospital.
Formal political power in Norway resides with its Parliament and government, while the monarch represents the nation with traditional duties ranging from state visits to national day celebrations.
Crown Prince Haakon will take over his father’s duties during the king’s sick leave, the palace said.
While some European monarchs, including in the Netherlands, have abdicated in recent years, handing the throne to a younger generation, King Harald has firmly ruled out stepping down, making clear his commitment is for life.
Born in 1937, the king is a great-great-grandchild of 19th century British Queen Victoria.