The socialist revolutionary’s death was announced by his brother, Raul Castro, the incumbent Cuban president, on state television late on Friday.
In his address the elderly leader said Mr Castro died at 10.29pm on Friday and he will be cremated on Saturday before a period of national mourning is observed.
He ended the announcement by shouting the revolutionary slogan: “Toward victory, always!”
Mr Castro stepped aside 10 years ago after suffering a severe gastrointestinal illness, and before his 90th birthday in August he told supporters he expected to die soon.
He led a guerrilla coup in 1959 to overthrow the regime of the US-backed former Cuban president Fulgencio Batista, and remained hostile to Washington throughout his life.
As US President Barack Obama moved to heal relations with Havana, Mr Castro responded: “We don’t need the empire to give us anything.”
When he closed the twice-a-decade congress of the Cuban Communist Party in April he called on his countrymen to maintain socialist ideals in the face of closer ties with the US.
Mr Castro’s last appearance in public was at an event to mark his birthday. The gala celebrated key moments in his life, including repelling the US-backed attempt to invade in the Bay of Pigs in 1961.
It was a defining moment in the Cold War, which reached its peak a year later when the world came to the brink of nuclear war during the Cuban Missile Crisis.
Mr Castro survived numerous assassination attempts by US spies, including a plot to kill him with an exploding cigar.
But he clung on to power, enduring decades under a crippling US trade embargo.
As its greatest ally, the Soviet Union, collapsed, Cuba remained a pariah Communist state at a cost of becoming one of the world’s poorest nations.
When his brother opened the door to a thawing of relations with the US in 2014, Castro cautiously blessed the deal - but only after a month-long silence.
Castro was born on August 13 1926, in eastern Cuba’s sugar country, where his father, a Spanish immigrant, organised labour for US sugar companies.
After attending Jesuit schools he received law and social science degrees from the University of Havana.
His first foray into violent subversion came in 1953 when he and Raul joined rebels in an attack on a military barracks in the eastern city of Santiago. Most of his comrades were killed and the brothers were jailed.
After receiving a pardon he fled to Mexico and raised a rebel force - supported by Che Guevara and others - who in 1956 sailed to Cuba, only for most to die in a botched landing.
But after rallying support in the country’s eastern mountains he led a revolutionary force into Havana and unseated Batista on New Year’s Day, 1959.
Declaring victory, he said: “I am not interested in power nor do I envisage assuming it at any time. All that I will do is to make sure that the sacrifices of so many compatriots should not be in vain, whatever the future may hold in store for me.”
Prominent figures have paid tribute to him.
They include vocal figures from the left wing of British politics.
Former London mayor Ken Livingstone said Mr Castro was an “absolute giant of the 20th century”, and blamed the US for the restrictions on civil liberties under his leadership.
He told Radio 4’s Today programme: “I’m sure they will, over time, move towards something like a traditional west European democracy. It could have happened a lot earlier if you hadn’t had, the entire time, a blockade by America, attempts to overthrow the regime, eight assassination attempts authorised by American presidents.”
Mr Livingstone said Cuba could reform now it was not under threat of American invasion “even if Trump goes a little bit bonkers”.
He admitted “of course Fidel did things that were wrong”, adding: “Initially he wasn’t very good on lesbian and gay rights, but the key things that mattered was that people had a good education, good healthcare and wealth was evenly distributed.
“He was not living as a billionaire laundering money off into a Panamanian bank account or anything like that, he was good for the people.”
Former Labour MP George Galloway tweeted a picture of himself with Castro, writing: “You were the greatest man I ever met Comandante Fidel. You were the man of the century. Hasta la Victoria Siepmre. Orden. RIP.”
He later added: “Who knew so many sickos were out there waiting for Fidel’s death? Dogs can dance on the lion’s grave. But they can never be a lion.”
Former Labour Cabinet minister and anti-apartheid leader Peter Hain, now Lord Hain, said: “Although responsible for indefensible human rights and free speech abuses, Castro created a society of unparalleled access to free health, education and equal opportunity despite an economically throttling USA siege.
“His troops inflicted the first defeat on South Africa’s troops in Angola in 1988, a vital turning point in the struggle against apartheid.”
Irish president Michael D Higgins was among a host of world leaders who also paid tribute, saying Castro guided Cuba “through a remarkable process of social and political change, advocating a development path that was unique and determinedly independent”.
He added: “Fidel Castro will be remembered as a giant among global leaders whose view was not only one of freedom for his people but for all of the oppressed and excluded peoples on the planet.”
Indian Prime Minister Narendra Modi said: “Fidel Castro was one of the most iconic personalities of the 20th century. India mourns the loss of a great friend. I extend my deepest condolences to the Government and people of Cuba on the sad demise of Fidel Castro. May his soul rest in peace.”
Mexican President Enrique Pena Nieto said: “Fidel Castro was a friend of Mexico, promoter of a bilateral relationship based on respect, dialogue and solidarity.”
Salvador Sanchez Ceren, President of El Salvador, tweeted: “With deep sorrow we received news of the death of my dear friend and eternal companion, Commander Fidel Castro Ruz.”