Israel has not commented on the death but many countries have warned that it could help turn the Gaza war into a regional conflict.
Here are some key reactions:
Iran leader vows 'harsh punishment'
Haniyeh had been in Tehran to attend the inauguration of President Masoud Pezeshkian and Iran's supreme leader Ayatollah Ali Khamenei vowed "harsh punishment" for Israel.
"With this action, the criminal and terrorist Zionist regime prepared the ground for harsh punishment for itself, and we consider it our duty to seek revenge for his blood as he was martyred in the territory of the Islamic Republic of Iran," Khamenei said in a statement carried by the IRNA official news agency.
Pezeshkian said that Iran "will defend its territorial integrity, honour, pride and dignity, and make the terrorist invaders regret their cowardly action."
Palestinian president calls killing 'cowardly'
Palestinian Authority president Mahmud Abbas is a rival of Haniyeh but slammed the killing as "a cowardly act and a serious escalation," his office said in a statement. "He urged our people and their forces to unite, remain patient, and stand firm against the Israeli occupation."
Qatar and Turkey warn of 'chaos'
Haniyeh was based in Qatar, which has been a mediator in the Gaza conflict, but also spent time in Turkey after going into exile. Qatar's foreign ministry called the killing a "heinous crime" and "shameful assassination".
The ministry said: "This assassination and the reckless Israeli behaviour of continuously targeting civilians in Gaza will lead to the region slipping into chaos and undermine the chances of peace."
Turkey's President Recep Tayyip Erdogan condemned what he called the "perfidious assassination" of his "brother" Haniyeh.
"This shameful act aims to sabotage the Palestinian cause, the glorious Gazan resistance and our Palestinian brothers' just fight, and to intimidate Palestinians," Erdogan added in a social media post.
US call for ceasefire
US Secretary of State Antony Blinken said that reaching a ceasefire in the Gaza war "is the enduring imperative" after the killing of Haniyeh. Germany stressed international calls for restraint to avoid "a regional conflagration", with a foreign ministry spokesman saying: "The logic of tit-for-tat reprisals is the wrong path."
Russia and China condemn
Haniyeh went to Moscow in September 2022 for talks on the Israel-Palestinian conflict while Hamas and rival Palestinian faction Fatah held talks in Beijing last week.
"It is a completely unacceptable political assassination, and this will lead to a further escalation of tensions," Russia's Deputy Foreign Minister Mikhail Bogdanov told the state-run RIA Novosti news agency.
Konstantin Kosachev, the vice-president of Russia's upper house Federation Council, predicted a "sudden escalation of mutual hatred" in the Middle East.
China's foreign ministry spokesman Lin Jian said: "We are highly concerned about the incident and firmly oppose and condemn the assassination."
Middle East shock
The killing reverberated across the Middle East. Egypt's foreign ministry said that Israel's "dangerous escalation" in recent days "risks igniting a confrontation in the region that could yield to dire security consequences". Iraq's foreign ministry also called Haniyeh's killing "a threat to security and stability in the region". Oman also condemned the action.
Hamas allies rally
Hamas allies in the region rallied around the Palestinian militants.
"The martyrdom of leader Haniyeh... will increase the determination and stubbornness of the mujahideen resistance fighters on all resistance fronts... and will make their resolve stronger in confronting the Zionist enemy," Lebanese militants Hezbollah, who are in a growing standoff with Israel, said in a statement.
The group described Haniyeh as "one of the great resistance leaders of our time who stood bravely against the American hegemony project and the Zionist occupation."
"Targeting him is a heinous terrorist crime and a flagrant violation of laws and ideal values," said Mohammed Ali al-Huthi, a political bureau member of Yemen's Huthi rebels, who have staged rocket attacks on Israel. Afghanistan's Taliban rulers called Haniyeh's death "a great loss".
