Foreign heads of state from more than 40 countries were among 36,000 people who witnessed the event complete with pomp, military honours and an airforce flyover at a rugby stadium in Pretoria.
The 66-year-old leader was unanimously elected by parliament to a five-year term after his African National Congress (ANC) won legislative elections on 8 May, taking 230 of the 400 seats.
The ANC collected 57.5 percent of the vote, its weakest result since apartheid was overturned 25 years ago.
Ramaphosa is a trade unionist who played a prominent part in the struggle to end white-minority rule before becoming a successful businessman.
He faces a herculean task to tackle the country's many problems - from a sickly economy in which more than a quarter of the workforce is jobless, to entrenched crime and corruption, not to mention land ownership with most valuable farmland still overwhelmingly in the hands of whites.