Haakon became Norway's monarch when King Harald V died on Friday at 89. He has adopted the motto "All for Norway," which dates back to his great-grandfather.
He was driven the short distance from the royal palace to the Storting, or parliament, in an open car. There, speaker Masud Gharahkhani read the official announcement of Harald's death and Haakon's succession. Haakon, 53, then took the oath.
"I promise and swear that I will govern the Kingdom of Norway in accordance with its Constitution and Laws; so help me God, the Almighty and Omniscient," he said.
Norway's monarchy has old roots, tracing its origins to King Harald Fairhair in the 9th century. When Norway regained its independence from Sweden in 1905, three-quarters of voters in a national plebiscite approved restoring the monarchy.
Today, its role is largely symbolic as power lies with the elected government and parliament, but the country's royal family retains a role in promoting the wealthy nation around the world.
On Monday, Harald's coffin was taken to the chapel of the royal palace in downtown Oslo, where it will remain until his funeral on September 9.
From Tuesday afternoon until September 8, the chapel will be open every day for the public to pay their last respects by filing past the coffin.
The royal palace said in a statement that visitors aren't allowed to take photos or film inside the chapel, and they can't leave flowers or other items.
Haakon's wife, Queen Mette-Marit, joined Monday's coffin procession in her first official appearance since assuming the title. She has been largely absent from the public eye following her lung transplant in June.
Ahead of Haakon's pledge of allegiance, the palace said that Mette-Marit wasn't going to the ceremony.
In a statement, it quoted Are Holm, a pulmonologist at Oslo's University Hospital, as saying complications following a transplant aren't uncommon and Mette-Marit has experienced them on several occasions, "including today."
"As a result, the Queen must remain under observation throughout the day. Unfortunately, the Queen will therefore be unable to attend the swearing-in ceremony at the Storting today," he said.
with DPA
