Ahmad Shah Massoud (Dari Persian: احمد شاه مسعود ) September 2, 1953 – September 9, 2001 was an Afghan political and military leader, who was a powerful military commander during the resistance against the Soviet occupation between 1979 and 1989 and in the following years of civil war. He was assassinated by Al Qaeda on September 9, 2001.

Massoud came from an ethnic Tajik, Sunni Muslim background in the Panjshir valley of northern Afghanistan. He began studying engineering at Polytechnical University of Kabul in the 1970s, where he became involved with fundamentalist Muslim anti-communist movements around Burhanuddin Rabbani, a leading Islamist. He was part of a Pakistan-backed failed rebellion against Mohammed Daoud Khan's government. After the Soviet occupation of 1979, his role as an insurgent leader earned him the nickname of "Lion of Panjshir" (شیر پنجشیر) among his followers. In 1992, after he disturbed the UN plan to install an interim government to replace that of President Mohammad Najibullah, he was appointed as the minister of defense through the Peshawar Accord, a peace and power-sharing agreement, in the post-communist Islamic State of Afghanistan. His militia fought to defend the capital against militias led by Gulbuddin Hekmatyar, Abdul Rasul Sayyaf, Abdul Ali Mazari, Abdul Rashid Dostum and eventually the Taliban, who started to lay siege to the capital in January 1995 after the city had seen fierce fighting; at least 60,000 civilians were killed, many more injured, public property, government offices and the Kabul Museum had been looted, and two thirds of the population had fled the city.

Following the rise of the Taliban in 1996, Massoud, who rejected the Taliban's fundamentalist interpretation of Islam, returned to armed opposition until he eventually fled to Kulob, Tajikistan, destroying the Salang Tunnel on his way north. He became the military and political leader of the United Islamic Front for the Salvation of Afghanistan (also known in the West as the Northern Alliance). He was assassinated, probably at the instigation of al-Qaeda, in a suicide bombing on September 9, 2001, just two days before the September 11 attacks in the United States which led to the North Atlantic Treaty Organisation invading Afghanistan, allying with Massoud's forces.

Massoud was posthumously named "National Hero" after the Taliban were ousted from power. The date of Massoud’s death, September 9, is observed as a national holiday known as "Massoud Day". His followers call him Amir Sāhib-e Shahīd (امیر صاحب شهید).