According to reports, the former Real Madrid star was rushed to Can Misses Hospital on Friday evening, August 9 after coming down with pneumonia.
Ronaldo owns property on Ibiza, a Spanish Mediterranean island, and is a frequent visitor.
The 41-year old is, however, said to be making good progress, with hospital sources at Misses saying they were unable to give out any information for "data protection reasons".
Several hours later Ronaldo asked to be transferred to a private hospital called Clinica Nuestra Senora del Rosario, the newspaper added. He is thought to have arrived on the island a few days ago before his illness started.
Ronaldo, who was on holiday in Ibiza when he was taken ill, retired from football in 2011 following a hugely successful career which featured stints at PSV Eindhoven, Barcelona, Inter Milan, AC Milan, Brazilian sides Corinthians and Cruzeiro as well as Real Madrid, where he is a club ambassador.More news: Liverpool Star Urges Reds To Show Pre-Season Results Against West Ham
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He hit eight goals and scored in every match in Brazil's 2002 World Cup win apart from the quarter-finals en route to lifting the trophy in Japan.
Ronaldo, whose full name is Ronaldo Luiz Nazario de Lima, rose to fame for his exploits as a striker for many of Europe's top clubs, as well as the Brazilian national team.
In his later years he battled with overweight because of a thyroid problem.
Pneumonia is swelling of the tissue in one or both lungs and is usually caused by a bacterial infection.
It can be life-threatening, especially among the elderly and those with underlying health conditions.