On 21 September 2013, four masked gunmen attacked the Westgate shopping mall, an upscale mall in Nairobi, Kenya. There are conflicting reports about the number killed in the attack, since part of the mall collapsed due to a fire that started during the siege. The attack resulted in 71 total deaths, including 62 civilians, five Kenyan soldiers, and four attackers. Approximately 200 people were wounded in the mass shooting.

The extremist Islamic group al-Shabaab claimed responsibility for the incident, which it characterised as retribution for the Kenyan military's deployment in the group's home country of Somalia. Many media outlets also suspected the insurgent group's involvement in the attack based on earlier reprisal warnings it had issued in the wake of Operation Linda Nchi from 2011 to 2012.

Kenyan authorities arrested dozens of people in the aftermath of the attack, but had not announced any suspects directly related to the siege. On 4 November 2013, a Kenyan court charged four Somali nationals with harbouring the gunmen in their homes, with each pleading not guilty.

On 20 September 2015, Foreign Policy magazine reported the Westgate attack on 21 September lasted several hours, with the last victim killed before special Kenyan security forces entered the mall. The mall was officially declared secured on 24 September.

Harakat al-Shabaab al-Mujahideen (HSM; Arabic: حركة الشباب المجاهدين, lit. 'Ḥarakat ash-Shabāb al-Mujāhidīn', Somali: Xarakada Mujaahidiinta Alshabaab, lit. 'Mujahideen Youth Movement' or 'Movement of Striving Youth'), more commonly known as al-Shabaab, is a Somalia-based terrorist jihadist fundamentalist group active in East Africa and Yemen. The group describes itself as waging jihad against "enemies of Islam" and is engaged in combat against the Federal Government of Somalia and the African Union Mission to Somalia (AMISOM). The group has been suspected of having links with al-Qaeda in Islamic Maghreb and Boko Haram. Due to its Wahhabi roots, al-Shabaab is hostile to Sufi traditions and has often clashed with the Somali Sufi militia Ahlu Sunna Waljama'a. Al-Shabaab’s leaders and commanders are mainly from the Hawiye clan, which is one of the largest clans in Somalia. It has attracted some members from Western countries, including Briton Samantha Lewthwaite and American Abu Mansoor Al-Amriki.

Formed in the mid-2000s, al-Shabaab came to prominence in the 2006–2009 Somalia War. In early August 2011, the Transitional Federal Government under Sharif Sheikh Ahmed and its AMISOM allies managed to capture all of Mogadishu from the al-Shabaab militants. In 2012, al-Shabaab pledged allegiance to the militant Islamist organization al-Qaeda and its leader Ayman al-Zawahiri. In February 2012, some of the group's leaders quarreled with al-Qaeda over the union and quickly lost ground. Al-Shabaab's troop strength was estimated at 7,000–9,000 militants in 2014. In August 2014, the Somali-government-led Operation Indian Ocean was launched to clean up the remaining insurgent-held pockets in the countryside. On 1 September 2014, a U.S. drone strike carried out as part of the operation killed al-Shabaab leader Ahmed Abdi Godane, also known as Mukhtar Abu Zubair. U.S. authorities hailed the raid as a major symbolic and operational loss for al-Shabaab, and the Somali government offered a 45-day amnesty to all moderate members of the militant group.As of 2015, the group had retreated from the major cities, however al-Shabaab still controlled large parts of the rural areas. The group remains strong and active and is responsible for many terrorist attacks with high death tolls, including the September 2013 Westgate shopping mall attack in Kenya, the 14 October 2017 Mogadishu bombings and the December 2019 Mogadishu bombing. Al-Shabaab has been designated as a terrorist organization by Australia, Canada, Malaysia, the United Arab Emirates, the United Kingdom and the United States. As of June 2012, the U.S. State Department had open bounties on several of the group's senior commanders. As of 2020, al-Shabaab had an estimated 5,000-10,000 fighters across Somalia.