Jacob Zuma married for the fifth time in early January
South Africa's main opposition party has accused the country's president of contradicting the government's message on HIV/Aids prevention.
President Jacob Zuma had a child with Sonono Khoza, the daughter of local world cup boss Irvin Khoza last year, claims the Sunday Times newspaper.
Mr Zuma has also been criticised for being polygamous, in a country with some five million HIV-positive people.
Mr Zuma, 67, married his third wife Thobeka Madiba-Zuma, 36, in January.
At the time, his office said he had 19 children.
So far, neither the ANC nor the government have commented on the story, splashed across the front page of South Africa's Sunday Times newspaper.
The newspaper alleges that a girl was born to Mr Zuma and Ms Khoza last October.
Ms Khoza is 39 years old and works for a bank.
MRS JACOB ZUMA
Thobeka Madiba-Zuma - married, January 2010 (above)
Nompumelelo Ntuli - married, January 2008
Sizakele Khumalo - married, 1973
Home Affairs Minister Nkosazana Dlamini-Zuma - divorced, 1998
Kate Mantsho Zuma - died, 2000
Her father is a friend of Mr Zuma and, as chairman of the World Cup Organising Committee, is one of the most powerful men in South African football.
The BBC's Jonah Fisher in Johannesburg says the president's many supporters will no doubt say this is a private matter.
But, if true, it is not the first time that Mr Zuma's actions have directly contradicted the government's HIV/Aids policy.
In 2006, while being acquitted of rape, Mr Zuma admitted that he had made a mistake by having unprotected sex.
The Democratic Alliance, South Africa's largest opposition party, said the latest allegations showed the president undermining the struggle against Aids by having unprotected sex with multiple partners.
Mr Zuma's colourful private life is never far from the headlines, our reporter says.
Earlier this month, he married for the fifth time - with Thobeka Madiba-Zuma becoming one of three current wives.
He is also engaged to be married to another woman.
Polygamy is permitted in South African law and is regarded as an important part of Zulu culture.