Sources told Diario de Ibiza that Ronaldo was recovering well.
"Friends, I had a strong bout of flu in Ibiza and had to be taken to hospital on Friday but everything is ok", the 41-year-old said in a message on his official Twitter account.
Brazilian World Cup victor and legendary striker Ronaldo has been admitted to an intensive care unit with pneumonia while on vacation in Ibiza, according to local reports.
Later that night, he requested transfer to the Nuestra Señora del Rosario hospital owned by Policlinica, a private health-care company.
He is believed to have arrived in Ibiza for a summer break a few days ago.
Ronaldo, who rose to global fame in the 1990s, and played for various clubs, including Barcelona, Inter Milan and AC Milan, is widely considered to be one of the best football players of all time.
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During his playing career, Ronaldo was famous for his blistering pace and phenomenal goal-scoring record.
No-one at the private hospital Ronaldo is now said to be a patient at could be contacted early on Sunday for comment.
His full name is Ronaldo Luis Nazario de Lima.
In his later years he battled with overweight because of a thyroid problem.
Pneumonia affects the lungs and is caused by bacterial infection. It can be potentially life-threatening if severe and is particularly risky for the elderly or those with pre-existing conditions.
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.