King Goodwill Zwelithini of the Zulu nation in South Africa has died in hospital where he was being treated for diabetes-related issues.
The king, 72, was the leader of South Africa's largest ethnic group and an influential traditional ruler.
He had been admitted to hospital in KwaZulu-Natal last week to monitor his ongoing diabetes condition.
The king's prime minister thanked South Africa for its "continued prayers and support in this most difficult time".
President Cyril Ramaphosa has said that he will be accorded a state funeral - normally reserved for presidents and former leaders.
The date has not been announced but in the meantime, national flags will fly at half-mast.
He will lie in state in the royal palace in Nongoma, about 300km (185 miles) from Durban where he was in hospital, for a "few days", according to his uncle, Chief Mangosuthu Buthelezi.
The straight-talking king
By Pumza Fihlani, BBC News, Johannesburg
King Zwelithini was a direct descendent of King Cetshwayo, who led the Zulu nation during the war with the British in 1879.
Throughout his 50 year-reign he was a strong advocate for preserving cultural identity.
As custodian of Zulu traditions and customs, he revived many cultural practices including Umhlanga - also known as the Reed Dance ceremony. Seen by some as patriarchal, the ceremony is said to be aimed at celebrating virginity among girls and young women and raising awareness about HIV/Aids in a province with some of the highest rates of infection.
South Africa's president has described him as a "much-loved visionary".
He will be remembered as a straight-talker, at times to the ire of some.
He was on occasion critical of the governing African National Congress (ANC), accusing it of mishandling the country.
In 2015, he controversially said foreign nationals must pack up and return to their countries - at the time his comments were blamed for fuelling attacks against foreigners in his province.
The king later said the comments had been taken out of context and described the attacks as "vile".
Five things about King Goodwill Zwelithini:
- Named successor to the throne at just 20 years old in 1968
- Not crowned until 1971 because he went into hiding after receiving death threats
- Role was ceremonial but still hugely influential
- Reigned as king of the Zulu nation for five decades
- Leaves behind six wives and 28 children
How influential was King Goodwill Zwelithini?
King Zwelithini was one of the most well-known monarchs on the continent and perhaps globally.
He ruled the Zulu nation under the Traditional Leadership clause of South Africa's republican constitution but his role was largely ceremonial.
He led the 11 million-strong Zulu nation - about 18% of South Africa's population - but he also has family links to the Swazi and Xhosa nations through marriage and is respected by other cultural groups too.
He also took his cultural influence internationally as over the years he travelled to the West to promote trade and tourism for his home province of KwaZulu-Natal.
What role did King Goodwill Zwelithini play as apartheid was ending?
During apartheid, a legalised form of racism in which white people were privileged above all others, King Zwelithini's role was not well known.
But Chief Buthelezi often used him and Zulu nationalism to shore up his political position as head of the Inkatha Freedom Party, a bitter rival of the ANC, which under Nelson Mandela led the fight against apartheid.
King Zwelithini helped to bring the Zulu nation into the new political system by persuading Chief Buthelezi and his party to take part in South Africa's first democratic elections in 1994.
After the end of apartheid, there were times when the Zulu king praised the apartheid government - he had said the National Party had built a strong economy and that the ANC government had destroyed the gains of the past.
Who will be Zulu king now?
It is not clear who among the king's 28 children will succeed him - this is something the Zulu royal family will be expected to advise on.