Al-Qaeda (; Arabic: القاعدة, romanized: al-Qāʿida, lit. 'the Base', IPA: [ælqɑːʕɪdɐ]) is a Sunni pan-Islamist militant organization led by Salafi jihadists who self-identify as a vanguard spearheading a global Islamist revolution to unite the Muslim world under a supra-national Islamic state known as the Caliphate. Its members are mostly composed of Arabs, but also include other peoples. Al-Qaeda has mounted attacks on civilian and military targets in various countries, including the 1998 United States embassy bombings, the 2001 September 11 attacks, and the 2002 Bali bombings; it has been designated as a terrorist group by the United Nations Security Council, the North Atlantic Treaty Organization (NATO), the European Union, and various countries around the world.

The organization was founded in a series of meetings held in Peshawar during 1988, attended by Abdullah Azzam, Osama bin Laden, Muhammad Atef, Ayman al-Zawahiri and other veterans of the Soviet–Afghan War. Building upon the networks of Maktab al-Khidamat, the founding members decided to create an organization named "Al-Qaeda" to serve as a "vanguard" for jihad. Following the withdrawal of the Soviets in 1989, bin Laden offered mujahideen support to Saudi Arabia in the Gulf War in 1990–1991. His offer was rebuffed by the Saudi government, which instead sought the aid of the United States. The stationing of U.S. troops in Saudi Arabia prompted bin Laden to declare a jihad against the House of Saud, whom he condemned as takfir (apostates from Islam), and against the US. During 1992–1996, al-Qaeda established its headquarters in Sudan until it was expelled in 1996. It shifted its base to the Taliban-ruled Afghanistan and later expanded to other parts of the world, primarily in the Middle East and South Asia.

In 1996 and 1998, bin Laden issued two fatāwā calling for U.S. troops to leave Saudi Arabia. Al-Qaeda conducted the 1998 United States embassy bombings in Kenya and Tanzania, which killed 224 people. The U.S. retaliated by launching Operation Infinite Reach, against al-Qaeda targets in Afghanistan and Sudan. In 2001, al-Qaeda carried out the September 11 attacks, resulting in nearly 3,000 fatalities, substantial long-term health consequences and damaging global economic markets. The U.S. launched the war on terror in response and invaded Afghanistan to depose the Taliban and destroy al-Qaeda. In 2003, a U.S.-led coalition invaded Iraq, overthrowing the Ba'athist regime which it wrongly accused of having ties with al-Qaeda. In 2004, al-Qaeda launched its Iraqi regional branch. After pursuing him for almost a decade, the U.S. military killed bin Laden in Pakistan in May 2011.

Al-Qaeda members believe a Judeo-Christian alliance (led by the United States) is conspiring to be at war against Islam and destroy Islam. As Salafist jihadists, members of Al-Qaeda believe that killing non-combatants is religiously sanctioned. Al-Qaeda also opposes what it regards as man-made laws, and wants to replace them exclusively with a strict form of sharīʿa (Islamic religious law, which is perceived as divine law). It characteristically organizes attacks such as suicide attacks and simultaneous bombing of several targets. Al-Qaeda's Iraq branch, which later morphed into the Islamic State of Iraq and Levant, was responsible for numerous sectarian attacks against Shias during its Iraqi insurgency. Al-Qaeda ideologues envision the violent removal of all foreign and secular influences in Muslim countries, which it denounces as corrupt deviations. Following the death of bin Laden in 2011, Al-Qaeda vowed to avenge his killing. The group was then led by Egyptian Ayman al-Zawahiri until his death in 2022. As of 2021, it has reportedly suffered from a deterioration of central command over its regional operations.