The Islamic State (IS), also known as the Islamic State of Iraq and the Levant (ISIL; ), Islamic State of Iraq and Syria (ISIS; ), and by its Arabic acronym Da'ish or Daesh (داعش, Dāʿish, IPA: [ˈdaːʕɪʃ]), is a militant Islamist group and former unrecognized quasi-state that follows the Salafi jihadist branch of Sunni Islam. It was founded by Abu Musab al-Zarqawi in 1999 and gained global prominence in 2014, when it drove Iraqi security forces out of key cities during the Anbar campaign, which was followed by its capture of Mosul and the Sinjar massacre. The organization significantly revamped the course of the Syrian civil war when it announced its expansion into Syria in mid-2013 and began conducting ground attacks against both Syrian government forces and Syrian opposition militias. By the end of 2015, it held an area that contained an estimated eight to twelve million people and stretched from western Iraq to eastern Syria, where it enforced its interpretation of Islamic law. ISIL was estimated at the time to have an annual budget of more than US$1 billion and more than 30,000 fighters.

Islamic State pledged allegiance to Al-Qaeda and participated in the Iraqi insurgency following the 2003 invasion of Iraq by a multi-national coalition led by the United States; under the name "Islamic State of Iraq". The group declared itself as the "Islamic State of Iraq and Levant" in 2013, as it sought the forced integration of Al-Nusra Front, Al-Qaeda's Syrian affiliate. Al-Nusra and AQ leaderships condemned the move, resulting in ISIL's split and subsequent conflict with the wider Al-Qaeda network. In 2014, the group proclaimed itself to be a worldwide caliphate, and began referring to itself as the Islamic State (الدولة الإسلامية, ad-Dawlah al-Islāmiyah). As a caliphate, it claimed religious, political, and military authority over Muslims worldwide. Its adoption of the name "Islamic State" and its idea of a caliphate have been criticised, with the United Nations, various governments, and mainstream Muslim groups rejecting its statehood and legitimacy.

In mid-2014, an international military coalition led by the United States intervened against ISIL in Syria as well as in Iraq with an airstrike campaign, in addition to supplying advisors, weapons, training, and supplies to ISIL's enemies in the Iraqi Armed Forces and the Syrian Democratic Forces. This campaign reinvigorated the latter two forces and damaged ISIL, killing tens of thousands of its fighters and reducing its financial and military infrastructure. The American-led intervention was followed in 2015 by a Russian military intervention exclusively in Syria, in which ISIL lost thousands more fighters to airstrikes, cruise missile attacks, and other Russian military activities, and had its financial base further degraded. In July 2017, the group lost control of its largest city, Mosul, to the Iraqi military, followed by the loss of its de facto political capital of Raqqa to the Syrian Democratic Forces. By December 2017, IS controlled just 2% of its maximum territory (achieved in May 2015). In December 2017, Iraqi forces had driven the last remnants of the group in that country underground, three years after it had captured about a third of Iraq's territory. By March 2019, IS lost one of their last significant territories in the Middle East in the Deir ez-Zor campaign, and effectively surrendered their "tent city" and pockets in Al-Baghuz Fawqani to the Syrian Democratic Forces after the Battle of Baghuz Fawqani.

The group has been designated as a terrorist organisation by the United Nations. It is well known for its videos of beheadings and other types of executions of both soldiers and civilians, including journalists and aid workers, as well as its destruction of cultural heritage sites. The international community holds IS responsible for committing massive human rights abuses, genocide, war crimes, and crimes against humanity. The group committed genocide against Yazidis and against Christians on a historic scale in northern Iraq and Syria, and systematically persecuted Shia Muslims during its rule. In October 2019, ISIL media announced that Abu Ibrahim al-Hashimi al-Qurashi had become the new leader of the group after Abu Bakr al-Baghdadi, the group's previous leader since 2013, died during an American military operation after detonating his suicide vest in Barisha, Syria. ISIL has also had a presence outside of the Middle East through its various "provinces" and affiliates, and has had a notable militant presence outside of the Arab world, predominantly in countries with significant or majority Muslim populations such as Nigeria, Cameroon, Chad, and Niger (West Africa Province); Afghanistan and Pakistan (Khorasan Province); as well as in countries with relatively low Muslim minority populations such as the Philippines (East Asia Province), the Democratic Republic of the Congo (Central Africa Province), and the Caucasus states (Caucasus Province).