The 29-year-old Real Madrid player started feeling unwell on Wednesday and was taken to hospital in Frankfurt, but medical tests showed nothing wrong with him, the Brazil Football Confederation (CBF) said.
The scare followed the Brazilians 1-0 win over Croatia in Berlin on Tuesday.
Ronaldo was expected to train normally with the rest of the Brazilian team on Thursday.
Ronaldo was the star of the last World Cup in Japan and South Korea, scoring both Brazilian goals in the 2-0 win over Germany in the final.
But his form has slumped over the last year as he has struggled with weight problems and he is expected to leave Madrid during the close season.
He was widely criticised for his lacklustre performance against the Croatians on Tuesday.
Fans have been unimpressed with
Ronaldo's recent performance
Coach Carlos Alberto Parreira, however, has said he intends to play him in the Brazilians' next game against Australia on Sunday.
Ronaldo famously played in the 1998 World Cup final in Paris against France despite being taken mysteriously taken ill shortly before the kick-off.
He needs to score just one more goal to equal the legendary Pele as Brazil's top all-time World Cup goalscorer.