Ahmad Shah Massoud (Dari/Pashto: احمد شاه مسعود, Persian pronunciation: [ʔæhmæd ʃɒːh mæsʔuːd]; September 2, 1953 – September 9, 2001) was an Afghan politician and military commander. He was a powerful guerrilla commander during the resistance against the Soviet occupation between 1979 and 1989. In the 1990s, he led the government's military wing against rival militias; after the Taliban takeover, he was the leading opposition commander against their regime until his assassination in 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 religious anti-communist movements around Burhanuddin Rabbani, a leading Islamist. He participated in a failed uprising against Mohammed Daoud Khan's government. He later joined Rabbani's Jamiat-e Islami party. During the Soviet–Afghan War, his role as a powerful insurgent leader of the Afghan mujahideen earned him the nickname "Lion of Panjshir" (شیر پنجشیر) among his followers, as he successfully resisted the Soviets from taking the Panjshir Valley. In 1992, he signed the Peshawar Accord, a peace and power-sharing agreement, in the post-communist Islamic State of Afghanistan. He was appointed the Minister of Defense as well as the government's main military commander. His militia fought to defend Kabul against militias led by Gulbuddin Hekmatyar and other warlords who were bombing the city, as well as later against the Taliban, who laid siege to the capital in January 1995 after the city had seen fierce fighting with at least 60,000 civilians killed.Following the rise of the Taliban in 1996, Massoud, who rejected the Taliban's fundamentalist interpretation of Islam, returned to armed opposition until he was forced to flee to Kulob, Tajikistan, strategically 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 or Northern Alliance, which by 2000 controlled only between 5 and 10 percent of the country. In 2001 he visited Europe and urged European Parliament leaders to pressure Pakistan on its support for the Taliban. He also asked for humanitarian aid to combat the Afghan people's gruesome conditions under the Taliban. Massoud was assassinated by two al-Qaeda assassins with a suicide bombing on September 9, 2001, ordered personally by the al-Qaeda leader Osama bin Laden himself. Two days later, the September 11 attacks occurred in the United States, which ultimately led to the North Atlantic Treaty Organisation invading Afghanistan and allying with Massoud's forces. The Northern Alliance eventually won the two-month-long war in December 2001, removing the Taliban from power.

Massoud has been described as one of the greatest guerrilla leaders of the 20th century and has been compared to Josip Broz Tito, Ho Chi Minh and Che Guevara. Massoud was posthumously named "National Hero" by the order of President Hamid Karzai 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 Amer Sāhib-e Shahīd (آمر صاحب شهید), which translates to "(our) martyred commander". He has been posthumously honored by a plaque in France in 2021, and in the same year was awarded with the highest honor of Tajikistan. However, he is also seen as a war criminal by others.