Al-Shahab, the radical Somali Islamist group, has joined ranks with al-Qaida, the terrorist network leader Ayman al-Zawahiri said in a video message.
The video, posted on Jihadist networks, saw al-Zawahiri boast about al-Qaida's expansion.
"I will break the good news to our Islamic nation, which will annoy the crusaders, and it is that the Shabab movement in Somalia has joined al-Qaida," Zawahiri said.
"The jihadist movement is growing and spreading within its Muslim nation despite facing the fiercest crusade campaign in history by the West."
Al-Shabab leader Ahmed Abdi Godane, also known as Mukhtar Abu Zubair, also appeared, saying to Zawahiri: ""We will move along with you as faithful soldiers.
"In the name of my mujahedeen brothers, leaders and soldiers I pledge obedience.
"Lead us on the road of jihad and martyrdom, in the footsteps that our martyr Osama bin Laden had drawn for us."
The al-Qaida leader also called on al-Shabab to treat Somalis with "leniency" and "humility".
The group should help them "solve their problems and fulfil their demands, especially those in need such as widows, orphans, the ill, the elderly and the poor". He urged a commitment to "spread justice".
"[Do] not forget their imprisoned brothers and sisters held in the jails of the corrupt and oppressive crusaders. Capture crusaders and Zionists wherever they can to exchange them with Muslim prisoners."
Al-Shabab militants are fighting to overthrow the weak but UN-backed Somali government. A bomb attack in the capital, Mogadishu, on 8 February killed at least 15 people and left more than 20 people injured.