Tests showed a “recurrence of a previous lung infection”, presidential spokesman Mac Maharaj said.
Mr Mandela is responding to his treatment, Mr Maharaj added.
News of the hospital stay has prompted much concern in South Africa.
The former president is revered at home and around the world for leading the struggle against white-minority rule before winning the country’s first democratic elections in 1994.
Despite being imprisoned for 27 years by the apartheid government, after his release he forgave his former enemies and urged South Africans of all races to work together and seek reconciliation.
He was awarded the Nobel Peace Prize in 1993.
He was last admitted to hospital in February when he was treated for abdominal pain.
In January 2011, he was treated for a serious chest infection.
Source: BBC
