The Israeli military said on Saturday that al-Qassam Brigades commander Izz al-Din al-Haddad was killed in what it described as a precise strike on Gaza City on Friday.
Israel has repeatedly carried out strikes on the enclave since the ceasefire started.
Hamas confirmed in a later statement that Haddad, who was born in 1970, was killed along with his wife and daughter.
It described him as a central figure in directing combat operations and accused Israel of trying to achieve politically through killings what it had failed to achieve militarily.
At Al Aqsa Martyrs Mosque in Gaza City, a joint funeral was held on Saturday for Haddad, his wife and their 19-year-old daughter.
Israel carried out at least two attacks on the Gaza Strip on Friday, killing seven Palestinians, including three women and one child, according to local medics.
A Palestinian source said Haddad was killed in an Israeli strike on an apartment building.
About 850 Palestinians have been killed in Israeli strikes since the October ceasefire, according to figures that do not distinguish between combatants and civilians.
Four Israeli soldiers were killed by militants during the same period.
Hamas does not disclose figures for casualties among its fighters.
In a joint statement with his defence minister on Friday announcing the military had targeted Haddad, Israeli Prime Minister Benjamin Netanyahu said Haddad was an architect of the October 7, 2023 attacks that precipitated Israel's assault on the Gaza Strip.
Haddad, who became the group's military chief after Israel's killing of Mohammad Sinwar in May 2025, "was responsible for the murder, abduction, and harm inflicted on thousands of Israeli civilians (and) soldiers," they said.
Nicknamed "the Ghost," Haddad had survived multiple assassination attempts by Israel, according to Hamas sources.
Israel's military says that he was one of Hamas' longest-serving commanders, rising through the ranks from the group's early establishment in the 1980s to hold several senior positions.
Israel and Hamas remain deadlocked in indirect talks to advance US President Donald Trump's post-war plan for the Gaza Strip that is meant to end more than two years of fighting.
Israel has stepped up attacks in the territory in the weeks since halting its joint bombing with the US in Iran, redirecting its fire back on the devastated Palestinian territory where the military says Hamas fighters are tightening their grip.
