"The Commission has received the requisite number of written communications supporting the admission of South Sudan to the African Union as the 54th union member state," the AU said in a statement.
Membership was granted after South Sudan submitted an official request to join the AU and became the first nation to be admitted to the body since 1993 when Eritrea was inducted.
Its AU membership means South Sudan will contribute to policy relating to security, economics, culture and democracy on the continent.
South Sudan gained independence on July 09 and became an official member of the United Nations on July 14.
There are several unresolved disputes with its northern neighbour Sudan, including striking a new oil deal and border demarcation.
Negotiations between the two nations are due to resume in Addis Ababa next week, led by AU mediator and former South African president Thabo Mbeki.