Hamas said in a statement that its leader Ismail Haniyeh was killed in his Tehran residence in an Israeli raid, the BBC reports.
The Palestinian group that controls Gaza said Haniyeh was "killed in a treacherous attack".
Israel has not yet responded or made any statement.
The group said he had attended the inauguration ceremony of new Iranian President Masoud Pezeshkian on Tuesday.
"The Hamas Islamic Resistance Movement mourns for our great Palestinian people, for the Arab and Islamic nation and for all the free people of the world," the statement said.
Most Hamas leaders keep a low public profile, while others have spent most of their lives dodging assassination attempts by Israel.
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