Charles and Queen Consort Camilla greeted President Ramaphosa for a ceremonial welcome at Horse Guards Parade in central London.
The monarch and Ramaphosa inspected the guard of honor together.
The parties then travelled to Buckingham Palace -- the route lined with the UK and South African flags -- in a carriage procession escorted by mounted soldiers from the Household Cavalry.
The two-day state visit is the first since Charles became king following the death of his mother, queen Elizabeth II, in September.
It comes more than a decade after the last by a South African leader, when Jacob Zuma came to the UK in 2010.
For Ramaphosa, a protegee of anti-apartheid icon Nelson Mandela, however, it also comes amid political difficulties and a threat of impeachment at home.
Ramaphosa also visited Westminster Abbey to lay a wreath at the Tomb of the Unknown Warrior and see the memorial stone for former South African President Nelson Mandela.
Addressing lawmakers in parliament, Ramaphosa lauded the two nations' close ties, forged in the wake of a difficult legacy of colonialism and apartheid.
Addressing UK parliamentarians in the ornate Royal Gallery of the Palace of Westminster, Ramaphosa nodded at the corruption endemic in his country -- including, for his critics, in the presidency itself.
He said his government was striving "to rebuild our economy in the wake of the pandemic and the era of state capture", as he urged the UK government to let in more South African students ahead of talks Wednesday with British Prime Minister Rishi Sunak.
Following the recent COP27 climate meeting in Egypt, Ramaphosa said an agreement to create a fund for poor and developing nations needed to be turned into cash.
"This should not be seen as charity," he said.
"It is compensation for the harm done -- and the harm yet to be done -- to people in developing economies as a consequence of the industrialiZation that wealthy countries have had over many years."
He later said he had discussed "initiatives that can be embarked upon by the Commonwealth under the leadership of his majesty the king" over lunch at Buckingham Palace.
He said the restructuring of South Africa's energy market to battle power shortages could provide opportunities for British companies.
"By pursuing these and other avenues for cooperation, I believe that we will define the relationship between South Africa and the United Kingdom in new and exciting ways," Ramaphosa said.
Sunak said ahead of his meeting with the South African president that he looked forward to discussing how to deepen the partnership between South Africa and the UK, "and capitalize on shared opportunities, from trade and tourism and security and defense," Sunak said in a statement ahead of the visit.
The last state visit to Britain by a South Africa leader was by President Jacob Zuma in 2010, when he was met by Charles and Camilla at the start of the trip.
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SA President Cyril Ramaphosa Visits Westminster Abbey