President Jacob Zuma told reporters the 94-year-old Mandela was admitted for tests and there was "no cause for alarm."
"The medical team is assured of our support as they look after and ensure the comfort of our beloved founding president of a free and democratic South Africa," Zuma said.
Zuma added the periodic medical attention was "consistent with" Mandela's age, CNN said.
Few details of Mandela's condition were released by the hospital. Mandela has been described as being frail with some memory issues and has largely been out of public view in recent years. The BBC said he has been described by visitors as generally being in good spirits.