First contingent of 1,000 peacekeepers will leave from end of December.
Peacekeepers withdrawal to continue till end of 2020.
Amisom troops contributing countries include Uganda, Kenya, Burundi, Ethiopia, and Djibouti.
The African Union Mission in Somalia (Amisom) will start withdrawing troops gradually from Somalia next month, an official has said.
The Special Representative of the chairperson of the African Union Commission (SRCC) for Somalia, Mr Francisco Madeira, said that the first contingent of 1,000 peacekeepers will leave from end of December.
He indicated that the initiative would continue till end of 2020, anticipating the building of the Somali National Army to assume the country’s security responsibilities.
Mr Madeira further said that the troops' withdrawal would be accompanied by movement of the AU forces in parts of the country.
Victory over Shabaab
“Troop movements shouldn't cause alarm,” said Mr Madeira.
"This is a process of realignment to effect the reduction in numbers and also begin the handover process of national security responsibilities to the Somali national security forces,” he added.
Somalia's information minister, Abdirahman Osman, said the country has won the military war against the Al-Qaeda-linked Al-Shabaab militants, leading the group to switch to terror tactics.
Speaking to the BBC after the AU announcement of its forces gradual pull out, Mr Osman said the militants would soon be totally defeated.
The first unit of Amisom peacekeepers was deployed in 2007 from Uganda. Since then, the number has increased to 22,000-strong force, helping Somalia to ward off the Al-Shabaab militants.
Other Amisom troops contributing countries are Kenya, Burundi, Ethiopia and Djibouti.
However, other reports indicated that the US military was concerned about the Somalia's security forces' capacity.
