Harakat al-Shabaab al-Mujahideen (HSM) (Arabic: حركة الشباب المجاهدين‎; Ḥarakat ash-Shabāb al-Mujāhidīn, "Mujahideen Youth Movement" or "Movement of Striving Youth"), more commonly known as al-Shabaab (Arabic: الشباب‎, "The Youth" or "The Boys"), is the Somalia-based cell of the militant Islamist group al-Qaeda, formally recognized in 2012. As of 2012, the outfit controls large swathes of the southern parts of the country, where it is said to have imposed its own strict form of Sharia law. Al-Shabaab's troop strength as of May 2011 is estimated at 14,426 militants. In February 2012, Al-Shabaab leaders quarreled with Al-Qaeda over the union, and quickly lost ground.

The group is an off-shoot of the Islamic Courts Union (ICU), which splintered into several smaller factions after its defeat in 2006 by the Somali Transitional Federal Government (TFG) and the TFG's Ethiopian military allies. Al-Shabaab describes itself as waging jihad against "enemies of Islam", and is engaged in combat against the TFG and the African Union Mission to Somalia (AMISOM). Alleging ulterior motives on the part of foreign organizations, group members have also reportedly intimidated, kidnapped and killed aid workers, leading to a suspension of humanitarian operations and an exodus of relief agents. The outfit has been designated a terrorist organization by several Western governments and security services.