Hamas said Sinwar, who masterminded the Oct. 7 attacks on Israel, would succeed Haniyeh as the head of the political bureau.
Sinwar, the top Gaza leader of Hamas, is suspected to be hiding out in a vast underground network of tunnels underneath the coastal strip.
Israel has been aggressively searching for him, but Sinwar has remained elusive. He is a key decisionmaker in talks between Hamas and Israel on a hostage release and ceasefire deal.
Rep. Mike Waltz (R-Fla.) told Fox News that Sinwar is "the most radical" leader in Hamas.
"He's made it very clear he's not interested in a two-state solution, he's only interested in wiping Israel off the face of the Earth," he said.
Haniyeh was killed last Wednesday after a bomb exploded at his guest residence in Tehran, where he was attending the inauguration of the country's new president.
Israel, which has neither confirmed nor denied its role behind the death of Haniyeh, has taken out several Hamas leaders since the war began in October.
That includes Mohammed Deif, the top Hamas military commander, who Israel confirmed last week was killed in a July strike.
Sinwar will take over the militant group after it has been severely weakened, including losing or suffering a degradation of more than a dozen battalions.
Jonathan Schanzer, senior vice president for research at the Foundation for Defense of Democracies, said Sinwar is "running the group from tunnels under Gaza while he’s being hunted by Israel."
"Hamas is unraveling," he wrote on X.
The death of Haniyeh, along with a top Hezbollah commander last week, has raised tensions in the Middle East as Iran and its proxies vow to respond.
Also Tuesday, a defense official confirmed that five American troops, along with two contractors, were injured in a Monday attack on a U.S. airbase in western Iraq.
All are expected to be in stable condition, but two of the wounded have been flown out of the airbase for further treatment.
The airbase was struck by two rockets. It's not clear who was behind it, but Iranian-backed groups appear to have resumed attacks on U.S. forces after pausing them in February in Iraq and Syria following massive American strikes that month.
Read more about the Middle East tensions at TheHill.com.
