The Izz ad-Din al-Qassam Brigades (Arabic: كتائب الشهيد عز الدين القسام, lit. 'Battalions of martyr Izz ad-Din al-Qassam'; also spelt Izzedine or Ezzedeen Al-Qassam Brigades; often shortened to Al-Qassam Brigades, IQB or EQB), named after Izz ad-Din al-Qassam, is the military wing of the Palestinian organization Hamas. Currently led by Mohammed Deif and his deputy, Marwan Issa, IQB is the largest and best-equipped group operating within Gaza today.

Created in mid-1991, it was at the time concerned with blocking the Oslo Accords negotiations. From 1994 to 2000, the Al-Qassam Brigades has claimed responsibility for carrying out a number of attacks against Israelis.

At the beginning of the Second Intifada, the group became a central target of Israel. The group's strength and its ability to carry out complex and lethal attacks surprised many observers. The Al-Qassam Brigades operated several cells in the West Bank, but most of them were destroyed by 2004 following numerous operations of the Israeli Defense Forces (IDF) in the region. In contrast, Hamas retained a forceful presence in the Gaza Strip, generally considered its stronghold. Yahya Sinwar, Hamas political leader in the Gaza Strip since February 2017, is a military leader in the Brigades in Gaza.

The Al-Qassam Brigades are explicitly listed as a terrorist organization by the European Union, Australia, New Zealand, Egypt, and the United Kingdom. Though not explicitly mentioning IQB, the United States and Canada have designated its parent entity, Hamas, as a terrorist organization; Brigade leader Mohammed Deif has also been classified as a Specially Designated Global Terrorist by the US under Executive Order 13224. As the Brigades undertake military activity on behalf of Hamas, "organized terrorist activities associated with Hamas can be reliably attributed to the Brigades."