South African President Cyril Ramaphosa was inaugurated in a stadium in the capital Pretoria.
The chairman of the African National Congress (ANC) pledged to fight corruption and revive the fragile economy.
More than 30,000 people gathered to witness the ceremony.
Mr. Ramaphosa was elected earlier this month by a majority of 57.5%, the smallest since the seizure of power 25 years ago.
Read more: South African President Ramaphosa praises "new era"