Zulu King Goodwill Zwelithini has died in hospital, Prince Mangosuthu Buthelezi announced on Friday morning.
"It is with the utmost grief that I inform the nation of the passing of His Majesty King Goodwill Zwelithini," he said in a short statement.
He said Zwelithini's health "took a turn for the worse" and that he died in the early hours of Friday morning.
"On behalf of the royal family, we thank the nation for your continued prayers and support in this most difficult time," the statement read.
OBITUARY | Zulu King Goodwill Zwelithini: The longest-serving ruler of the Zulu people
Zwelithini had been ill for about a month before his death.
There were concerns about his blood sugar levels and he received treatment in the ICU when he became unstable.
His condition was treated with "caution" because of risks associated with Covid-19, Buthelezi previously said.
In early February, Buthelezi said rumours that Zwelithini had died were "cruel" and "irresponsible".
In December 1971, just after his 21st birthday, Zwelithini became the eighth king of the Zulu nation, succeeding his father, King Cyprian Bhekuzulu kaSolomon.
President Cyril Ramaphosa said he was "deeply saddened by the news of the passing of Isilo Samabandla, King Goodwill Zwelithini KaBhekuzulu".
He remembered the king "as a much-loved, visionary monarch who made an important contribution to cultural identity, national unity and economic development in KwaZulu-Natal and through this, to the development of our country as a whole".