The 87-year-old had been hospitalised for an infection while on a private trip in the resort island of Langkawi. He received a temporary pacemaker on Saturday.
The monarch was due to be transferred to a hospital in Oslo.
"He will stay in the hospital for a few days for treatment and rest," the royal household said in a statement on Sunday.
It was previously said that the king was expected to be on sick leave for two weeks.
Crown Prince Haakonhas been presiding in his father's absence.
King Harald, Norway's ceremonial head of state since 1991, is Europe's oldest living monarch. He has repeatedly been hospitalised with infections in recent years and has undergone heart surgery.
