The 89-year-old king was taken to the National Hospital in Oslo after attending two official events earlier in the day.
According to Norwegian media, the royal court said he was driven there in a private car and denied previous media reports that he was taken to hospital by ambulance.
King Harald was hospitalised in February during a holiday in Tenerife. He had suffered a skin infection in his leg and dehydration.
In March 2024, he received a pacemaker after falling ill during a holiday in Malaysia.
At the end of May this year, Queen Sonja was hospitalised after heart problems. She also has a pacemaker. Princess Astrid, the king's sister, has also been hospitalised during the year.
