His killing marks a huge success for Israel and a pivotal event in the year-long conflict. Western leaders said his death offered an opportunity for the war to end, but Israeli Prime Minister Benjamin Netanyahu said it would go on.
The Israeli military said it had killed Sinwar in an operation in the southern Gaza Strip on Wednesday.
"After completing the process of identifying the body, it can be confirmed that Yahya Sinwar was eliminated," it said.
There was no immediate comment from Hamas, but sources in the group said that indications from Gaza suggested Sinwar had been killed in an Israeli operation.
In Israel, families of hostages held by Hamas in Gaza said they hoped for a ceasefire now to bring home the captives but also feared their loved ones were in greater danger.
In Gaza, pounded relentlessly by Israeli forces for a year, residents said they believed the war would continue, but they clung to their hope of self-determination.
U.S. President Biden, who spoke to Netanyahu by phone to congratulate him, as well as French President Emmanuel Macron, said Sinwar's death provided a chance for the more than year-long conflict in Gaza to finally end and for Israeli hostages to be brought home.
The U.S. wants to kickstart talks on a proposal to achieve a ceasefire and secure the release of hostages, U.S. State Department spokesperson Matthew Miller said, calling Sinwar the "chief obstacle" to ending the war.
"That obstacle has obviously been removed. Can’t predict that that means whoever replaces (Sinwar) will agree to a ceasefire, but it does remove what has been in recent months the chief obstacle to getting one," he said. In recent weeks, Sinwar had refused to negotiate at all, Miller said.
Netanyahu, speaking in Jerusalem just after the death was confirmed, said Sinwar's death offered the chance of peace in the Middle East, but warned that the war in Gaza was not over and Israel would continue until its hostages were returned.
"Today we have settled the score. Today evil has been dealt a blow but our task has still not been completed," Netanyahu said in a recorded video statement. "To the dear hostage families, I say: This is an important moment in the war. We will continue full force until all your loved ones, our loved ones, are home."
Israeli Foreign Minister Israel Katz said: "This is a great military and moral achievement for Israel."
He called Sinwar a "mass murderer who was responsible for the massacre and atrocities of Oct. 7" - the Hamas-led attack on Israel that unleashed the assault on Gaza.
The head of Israel's military, Lieutenant General Herzi Halevi, said Israel's pursuit of Sinwar over the past year had driven him "to act like a fugitive, causing him to change locations multiple times."
He said soldiers had come upon Sinwar during a regular operation without knowing he was there, unlike other operations against leaders based on comprehensive intelligence.
The killing occurred during a ground operation in the city of Rafah in southern Gaza during which Israeli troops killed three militants and took their bodies, Israel's Army Radio said.
RUTHLESS ENFORCER
Sinwar, who was named as Hamas' overall leader following the assassination of political chief Ismail Haniyeh in Tehran in July, was believed to have been hiding in the warren of tunnels Hamas has built under Gaza over the past two decades.
Despite Western hopes of a ceasefire, his death could dial up hostilities in the Middle East where the prospect of an even wider conflict has grown. Israel has launched a ground campaign in Lebanon over the past month and is now planning a response to an Oct. 1 missile attack carried out by Iran, ally of Hamas and Lebanon's Hezbollah.
But the demise of the man who planned the attack last year in which fighters killed 1,200 people in Israel and captured more than 250 hostages, according to Israeli tallies, could also help push forward stalled efforts to end the war in which Israel has killed more than 42,000 Palestinians, according to Gaza health authorities.
A ruthless enforcer once tasked with punishing Palestinians suspected of informing for Israel, Sinwar, who was born in 1962, made his name as a prison leader.
He emerged as a street hero in Gaza after serving more than 20 years in an Israeli jail for masterminding the abduction and murder of two Israeli soldiers and four Palestinians.
He was freed in 2011 as part of an exchange of more than 1,000 prisoners for one kidnapped Israeli soldier held in Gaza. Sinwar then quickly rose to the top of the Hamas ranks. He was dedicated to eradicating Israel.
Israel has killed several commanders of Hamas in Gaza as well as senior figures of Iran-allied Hezbollah in Lebanon, dealing heavy blows to its arch-foes.
HOSTAGES FATE
The killing raises new questions about the fate of the hostages still in Hamas' captivity. Sinwar was involved in negotiations that could have led to their release.
Families of Israeli hostages said that while the killing of Sinwar was a significant achievement, it would not be complete while hostages are still in Gaza.
Avi Marciano, the father of Noa Marciano, who was killed in captivity by Hamas, told Israeli broadcaster KAN that "the monster, the one who took her from me, who had the blood of all our daughters on his hands, finally met the gates of hell."
"A little justice, but no comfort," he said. "There will be comfort only when Naama, Liri, Agam, Daniela and Karina, our girls' friends, return home."
In Khan Younis in the south of the Gaza Strip, a displaced Palestinian named Thabet Amour told Reuters that the Palestinian fight would continue.
"This is resistance that does not disappear when men disappear," he said. "The assassination of Sinwar will not lead to the end of the resistance or to a compromise or surrender and raising the white flag."
