Barack Obama is to deliver the annual Nelson Mandela lecture in South Africa in a rare public appearance since he stepped down as US president.
His speech will coincide with events to mark 100 years since the birth of former South Africa President Nelson Mandela, who died in 2013 aged 95.
Both men were the first black presidents of their countries.
Mr. Obama has said he was "one of the countless millions who drew inspiration from Nelson Mandela's life".
As a student, he called the fight against white minority rule in South Africa "a struggle that touches each and every one of us", and encouraged his university to drop its investments in the country.
The eagerly awaited speech by Mr. Obama will be live-streamed from Johannesburg at 14:45 local time (12:45GMT) by the Nelson Mandela Foundation.
Its focus will be "creating conditions for bridging divides, working across ideological lines, and resisting oppression and inequality".
Previous speakers at the event include US entrepreneur and philanthropist Bill Gates, French economist Prof Thomas Piketty, former Liberian President Ellen Johnson-Sirleaf, Kenyan Nobel laureate and political activist Wangari Maathai, ex-UN Secretary General Kofi Annan and former US President Bill Clinton.
"We thought to ourselves: 'Who can best represent the legacy of Madiba?" said Nelson Mandela Foundation head Sello Hatang when announcing Mr. Obama as this year's choice.
He added: "Who took the baton when he became president of his own country? Who would be able to deal with issues of democracy in a world ripped apart by corruption?' We needed an African person."
Since its beginning in 2003, global leaders have used the lecture to speak about issues affecting South Africa, the continent, and the world.
Mr. Obama and his family spent eight days in Tanzania's famous Serengeti National Park before Mr Obama travelled to Kenya at the weekend to visit his ancestral home and now to South Africa.