The Real Madrid player, 25, was reportedly confirmed to have become a dad, two days after Portugal played Ivory Coast in the World Cup but did not reveal the identity of the mother of his child.
His youngest sister, Katia Aveiro, who is currently in Vilamoura, in the Algarve, confirmed the birth to the Portuguese newspaper Diario de Noticias.
"He is very happy, of course, who wouldn't be happy to have children?" the Daily Express quoted her as saying.
She said the baby boy has dark eyes and hair like his father.
Aveiro told the newspaper that Ronaldo would make an official announcement after he had returned from Madrid, where he is dealing with professional matters.
However, she did not reveal the baby's name, nor her mother's identity.
"The news is true, but he doesn't want to talk about it. It's his personal life and when he wants to, he will talk," she said, according to ANI reports.
Earlier, according to reports, there were rumors that the baby's mother is believed to be American but her identity has not been revealed.
A statement posted on what is billed as Ronaldo's official Facebook page said: "It is with great joy and emotion that I inform I have recently become father to a baby boy. As agreed with the baby's mother, who prefers to have her identity kept confidential, my son will be under my exclusive guardianship."