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The prime minister met pupils at a Cape Town school at the start of her trip
Theresa May has announced plans to boost Britain's investment in Africa after Brexit, during her first trip to the continent as prime minister.
In a speech in Cape Town, she pledged £4bn in support for African economies, to create jobs for young people.
She also pledged a "fundamental shift" in aid spending to focus on long-term economic and security challenges rather than short-term poverty reduction.
She will also visit Nigeria and Kenya during the three-day trade mission.
On her way to South Africa, the prime minister also played down warnings from the chancellor about the economic damage a no-deal Brexit could cause.
Talking to journalists on board RAF Voyager on Tuesday morning, Mrs May reiterated that she believed a no-deal Brexit was still better than a bad dealadding no-deal "wouldn't be the end of the world".
Last week Chancellor Philip Hammond warned in a letter that a no-deal Brexit could damage the economy.
Mrs May's tripwhich will see her meet the presidents of all three countriesaims to deepen economic and trade ties with growing African economies ahead of Britain leaving the EU in 2019.
Arriving in South Africa on Tuesday morning, Mrs May said she she wanted the UK to overtake the United States to become the G7's biggest investor in Africa by 2022.
Promising an extra £4bn in state investmentwhich she expects to be matched by the private sectorshe said while the UK could not match the "economic might" of some foreign investorssuch as China or the United Statesit offered a unique "high quality and breadth" of investment.
She defended the UK's aid spending in Africa, a target of criticism from some Tory MPs, saying it had "worked" to give millions of children and women an education and immunise millions against deadly diseases.
But she said she was "unashamed" that it had to work in the UK's own interest and pledged a new approach in future, focusing on helping British private sector companies invest in fast-growing countries like Cote D'Ivoire and Senegal while supporting countries, such as Chad, Mail and Niger, in "the frontline of instability".
The UK's overseas aid budget totalled £13.9bn in 2017, an increase of £555m in 2016.
UK direct investment in Africa was £42.7bn in 2016, compared to £44.3bn from the United States, £38bn from France and £31bn from China, according to data from the United Nations Conference on Trade and Development.
Mrs May said national self-interest and global co-operation were not in conflict and the UK could play a key role in harnessing the "innovation and creativity" of young people in Africa, 60% of whose population is under the age of 25%.
"The challenges facing Africa are not Africa's alone," she said. "It is the world's interest to see these jobs created."
BBC political correspondent Ben Wright said like other European leaders the PM saw "huge economic potential to tap" in Africa and was keen to support economic stability in the region in order to "stem the flow of migration".
UK no longer domit
The UK's historical relationship with many African countries still counts for something, but, as Prime Minister Theresa May will find on her trip to the continent, the UK now vies for attention with larger economies offering greater riches.
The continent's leaders need to decide who to prioritise: an ambitious but friendly China, the huge European Union bloc, the potential riches of the United States, or the historically-linked United Kingdom.
The prime minister's trip comes a week before the huge Forum on China-Africa Cooperation in Beijing. Dozens of African heads of state are expected there and China may offer new trade and finance deals.
Mrs May's trip seems rather low key in comparison.
Tunji Andrews, an economist based in Nigeria's commercial hub Lagos, says the UK has lost its once domit place in Africa's largest economy.
"While Britain remains a viable trade partner, it just doesn't hold the same value to Africa as China and to a lesser extent, the United States."
Security issues will also feature on the PM's agenda and she is expected to discuss the threat of Boko Haram in Nigeria, and the role of British troops based in Kenya who are helping countries fight al-Shabab militants in Somalia.
Mrs May's visit to Nairobi will mark the first by a UK prime minister to Kenya since Margaret Thatcher in 1988.
It is also the first to Sub-Saharan Africa by a British leader since David Cameron in 2013 for Mr Mandela's memorial service.
During her time in South Africa, Mrs May also presented a World War One reliclinked to one of the worst maritime disasters in English watersto South African President Cyril Ramaphosa.
The SS Mendi sank off the Isle of Wight in 1917 killing more than 600 South Africans en route to the Western Front to support British troops.
The ship's bell was given to BBC reporter Steve Humphrey in 2017 in a plastic bag at Swanage Pier, Dorset, after an anonymous phone calland will now be handed back by Mrs May.
The PM's diary
TuesdayMrs May will fly into Cape Town where she will meet young people, before delivering a keynote speech on trade and how UK private sector investment can be brought into Africa.
After a bilateral meeting with South African President Cyril Ramaphosa, she is expected to visit Robben Island, where Nelson Mandela was imprisoned.
WednesdayMrs May intends to meet Nigerian President Muhammadu Buhari in the capital Abuja before meeting victims of modern slavery in Lagos.
ThursdayIn Kenya, Mrs May will meet president Uhuru Kenyatta before visiting British troops and a business school. A state dinner hosted by Mr Kenyatta will conclude the trip.