Former Inter and Milan star Ronaldo has been in hospital in Ibiza since Friday after suffering from pneumonia.
The retired striker turns 42 next month and has struggled with weight issues since hanging up his boots in 2011.
Sources from local authorities in Ibiza and Formentera confirmed to Diario de Ibiza that Ronaldo was brought in to the Hospital de Can Misses de Ibiza on Friday evening.
Diagnosed with pneumonia, Il Fenomeno was then transferred to a private hospital in the ICU of the Policlinical Nuestra Senora del Rosario.
Ronaldo was spending his summer vacation in Ibiza and owns a house in the area.
During his career, the World Cup winner also played for Cruzeiro, PSV Eindhoven, Barcelona, Real Madrid and Corinthians.