Sinwar, who until now was the movement's Gaza chief, replaces Ismail Haniyeh, whose killing in Tehran last week sent tensions soaring across the Middle East and raised fears of a coordinated attack on Israel by Iran and its regional proxies.
By choosing him as the group's chief Hamas "is sending a strong message to the occupation that Hamas continues its path of resistance", a senior Hamas official told AFP.
Sinwar stands accused of masterminding the group's October 7 attacks, the worst in Israel's history, which left 1,198 dead and 251 taken hostage according to an AFP tally and Israeli official figures.
In the aftermath of that attack, the Israeli military insisted he was a "dead man walking", although Sinwar has not been seen since.
The October 7 attack was probably a year or two in the planning, "took everyone by surprise" and "changed the balance of power on the ground", said Leila Seurat of the Arab Centre for Research and Political Studies (CAREP) in Paris.
The ascetic 61-year-old is a security operator "par excellence", according to Abu Abdallah, a Hamas member who spent years alongside him in Israeli jails.
"He makes decisions in the utmost calm, but is intractable when it comes to defending the interests of Hamas," Abu Abdallah told AFP in 2017, after his former co-detainee was elected Hamas's leader in Gaza.
Punishing collaborators
After October 7, Israeli military spokesman Lieutenant Colonel Richard Hecht called Sinwar the "face of evil" and declared him a "dead man walking".
Born in the Khan Yunis refugee camp in southern Gaza, Sinwar joined Hamas when Sheikh Ahmad Yassin founded the group around the time the first Palestinian intifada began in 1987.
Sinwar set up the group's internal security apparatus the following year and went on to head an intelligence unit dedicated to flushing out and mercilessly punishing -- sometimes killing -- Palestinians accused of providing information to Israel.
According to a transcript of an interrogation with security officials published in Israeli media, Sinwar professed to have strangled an alleged collaborator with a keffiyeh scarf in a Khan Yunis cemetery.
A graduate of the Islamic University in Gaza, he learned perfect Hebrew during his 23 years in Israeli jails and is said to have a deep understanding of Israeli culture and society.
He was serving four life terms for the killing of two Israeli soldiers when he became the most senior of 1,027 Palestinians released in exchange for Israeli soldier Gilad Shalit in 2011.
Sinwar later became a senior commander in the Ezzedine al-Qassam Brigades, the military wing of Hamas, before taking overall leadership of the movement in Gaza.
While his predecessor, Haniyeh, had encouraged efforts by Hamas to present a moderate face to the world, Sinwar has preferred to force the Palestinian issue to the fore by more violent means.
The health ministry in Hamas-run Gaza says Israel's withering aerial and ground assault launched in response to the October 7 attacks has killed at least 39,653 people in the Palestinian territory.
'Radical and pragmatic'
Sinwar dreams of a single Palestinian state bringing together the Gaza Strip, the occupied West Bank -- controlled by Mahmud Abbas's Fatah party -- and annexed east Jerusalem.
According to US think-tank the Council on Foreign Relations, he has vowed to punish anyone obstructing reconciliation with Fatah, the rival political movement with which Hamas engaged in factional fighting after elections in 2006.
That coming together remains elusive, but the prisoner releases resulting from the brief November truce agreement with Israel have seen Hamas's popularity soar in the West Bank.
Sinwar has pursued a path of being "radical in military planning and pragmatic in politics", according to Seurat.
"He doesn't advocate force for force's sake, but to bring about negotiations" with Israel, she said.
The Hamas chief was added to the US list of the most wanted "international terrorists" in 2015.
Security sources outside Gaza say that Sinwar has taken refuge in the network of tunnels built under the territory to withstand Israeli bombs.
Vowing in November to "find and eliminate" Sinwar, Israeli Defence Minister Yoav Gallant urged Gazans to turn Sinwar in, adding "if you reach him before us, it will shorten the war".
