He died months before he was to step down after 15 turbulent years as the country's leader.
Burundi's President Pierre Nkurunziza, aged 55, has died suddenly after suffering heart failure, the government said on Tuesday.
Nkurunziza first began to feel unwell after attending a volleyball match on Saturday, and appeared to be improving until he took a turn for the worse on Monday, when his heart stopped, the presidency said in a statement.
"Despite intense efforts, the medical team was not able to save the patient," said the statement, adding that doctors had spent several hours trying to revive him.
His death comes two months before he was due to step down after 15 years as the country's leader.
The announcement in 2015 that he would run for a third term plunged the country into chaos, leading to protests and a failed coup.
Following his successful election, the country has been gripped by fear with rights groups regularly denouncing crackdowns on the opposition and the media.
After May's election he decided to retire, though could have run for another term due to changes in the constitution.
His hand-picked successor Evariste Ndayishimiye was elected as his replacement last month.
Nkurunziza came to power in 2005 and fought for the ethnic Hutu rebellion during the country's 1993-2006 civil war with the minority Tutsi-dominated army.
The civil war left some 300,000 people dead.
Nkurunziza's wife, first lady Denise Bucumi, is currently recovering from the coronavirus in a Nairobi hospital after being evacuated late last month.
The country will now observe seven days of mourning to mark his death, according to the statement from the presidency.