Yahya Ibrahim Hassan Sinwar (Arabic: يحيى إبراهيم حسن السنوار, romanized: Yaḥyá Ibrāhīm Ḥasan al-Sinwār; 29 October 1962 – 16 October 2024) was a Palestinian militant and politician who served as fourth chairman of the Hamas Political Bureau from August 2024, and as the second leader of Hamas in the Gaza Strip from February 2017, succeeding Ismail Haniyeh in both roles. He was killed in a clash with the Israel Defense Forces (IDF) in October 2024.

Sinwar was born in the Khan Yunis refugee camp in Egyptian-occupied Gaza in 1962 to a family who had been expelled or fled from Majdal 'Asqalan during the 1948 Palestine War. He finished his studies at the Islamic University of Gaza, where he received a bachelor's degree in Arabic studies. In 1989, Sinwar was sentenced to four life sentences in Israel for orchestrating the abduction and killing of two Israeli soldiers and four Palestinians he considered to be collaborators. He spent 22 years in prison until his release among 1,026 others in a 2011 prisoner exchange for Israeli soldier Gilad Shalit. During his time in prison, Sinwar continued to coordinate the military activities of Hamas. Sinwar was one of the co-founders of the security apparatus of Hamas.

In 2017, Sinwar was elected as the leader of Hamas in Gaza and claimed to pursue "peaceful, popular resistance" the following year, supporting the 2018–2019 Gaza border protests, though he was also reported to have been dedicated to eradicating Israel and was said to have seen military confrontation as the only path to "liberating Palestine", saying that this would be achieved "by force, not negotiations". He also developed strong ties with Iran. Re-elected as Hamas leader in 2021, Sinwar survived an assassination attempt by Israel that same year. He was widely regarded as the mastermind behind the October 7 attacks in 2023, which was followed by the Gaza war that spilled over to other parts of the Middle East.

Hamas and the Izz ad-Din al-Qassam Brigades have been designated terrorist organisations by the United States, the European Union, and other countries, and in September 2015, Sinwar was specifically designated a terrorist by the United States government. In May 2024, Karim Khan, the prosecutor of the International Criminal Court, announced his intention to apply for an arrest warrant for Sinwar for war crimes and crimes against humanity, as part of the ICC investigation in Palestine.

Yahya Ibrahim Hassan al-Sinwar was born on 29 October 1962, in the Khan Yunis refugee camp, when the Gaza Strip was under Egyptian occupation, where he spent his early years. His family were forcibly expelled from Majdal Asqalan (Arabic: مدينة المجدل, romanized: Medīnat al-Majdal), now known as Ashkelon, during the Nakba, and sought refuge in the Gaza Strip. Sinwar, discussing his refugee upbringing, tied it to his Hamas involvement in conversations with fellow prisoners during his later imprisonment. According to Esmat Mansour, another inmate, Sinwar was deeply affected by the communal living conditions and food distribution in the refugee camp. After he graduated from high school at Khan Yunis Secondary School for Boys, he went on to the Islamic University of Gaza, where he received a bachelor's degree in Arabic studies. His younger brother is Mohammed Sinwar, a military leader of Hamas. Sinwar was a hafiz (a title given to those who have completely memorized the Quran).

Sinwar was first arrested in 1982 for subversive activities and he served several months in the Far'a prison where he met other Palestinian activists, including Salah Shehade, and dedicated himself to the Palestinian cause. Arrested again in 1985, upon his release he co-founded with Rawhi Mushtaha the Munazzamat al Jihad w'al-Dawa (Majd), an organization that worked, among others, to identify collaborators with Israel among the Palestinian population, which in 1987 became the Hamas "police". Sinwar's killing of suspected collaborators with Israel gained him the nickname "The Butcher of Khan Younis".

When asked about this nickname in The New Yorker's August 2024 profile on Sinwar, Basem Naim, a member of Hamas's leadership, said: "I think this is nonsense. That is the first time I have ever heard this." He and other Hamas leaders and supporters added that "Israelis require a great villain" and made one out of Sinwar for this reason. Academic expert on Hamas Khaled Hroub said Sinwar is "widely respected as a great organizer", and that claims of his alleged ruthlessness had not been proved. "Before October 7th, I hadn't heard all these terrible stories", Hroub said, adding: "I think some of these stories came about to complete this image of Sinwar the villain. He is decisive, that is true, and maybe people started to extrapolate from that and spice it up."

In 1988, Sinwar planned the abduction and killing of two Israeli soldiers and the murder of four Palestinians whom he suspected of cooperating with Israel. He was arrested on February that year; during questioning he admitted to strangling one of the victims with his bare hands, suffocating another with a keffiyeh, shooting a third and choking and punching a fourth. He was sentenced to four life sentences in 1989. Sinwar regarded extracting confessions from collaborators as a righteous obligation. He told interrogators that one of them had even said "he realized he deserved to die." Sinwar persisted in targeting informants while in prison. Israeli authorities suspected him of ordering the beheadings of two suspected informants. Hamas operatives reportedly disposed of the victims' severed body parts by throwing them out of cell doors and telling guards to "take the dog's head."