The inauguration followed his ruling African National Congress party’s 57.5% victory in this month’s election. It was the party’s weakest election showing since the ANC took power at the end of the harsh system of racial apartheid in 1994.
Mr Ramaphosa first took office last year after former President Jacob Zuma was pressured to resign amid corruption scandals that badly damaged public faith in the ANC.
There was no sign at Saturday’s ceremony of Mr Zuma, who has insisted he did nothing wrong and that allegations are politically motivated. His allies within the ANC leadership pose a challenge to Mr Ramaphosa as he pursues reforms.
Ahead of the election, Mr Ramaphosa apologised to South Africans for the political turmoil. He also vowed to continue the fight against mismanagement and graft that has hurt the country’s economy, the most developed in sub-Saharan Africa.