Former Brazil star Ronaldo was admitted to hospital in Ibiza last Friday with a case of pneumonia.
Newspaper Diario de Ibiza said the two-time Ballon d'Or victor and 2002 World Cup Golden Boot victor is now in the intensive care unit at the private Policlinica Nuestra Senora del Rosario hospital on the Spanish island, having been taken to a public hospital on the island on Friday evening.
Respected island daily Diario de Ibiza said he had asked for a transfer to a private hospital called Clinica Nuestra Senora del Rosario just before midnight the same day. The hospital where Ronaldo is recovering could not be reached by Reuters for comment.
The former Real Madrid forward, who also played for Barcelona, Inter Milan and AC Milan, thanked his fans for their love and well wishes.
Pneumonia is swelling of the tissue in one or both lungs and is usually caused by a bacterial infection.
Ronaldo is said to own property on the Spanish Island and is believed to be visiting on holiday.
Since retirement, however, he has battled thyroid problems and gained significant weight.
In total, Ronaldo scored 15 goals at World Cups and won the competition's Golden Boot award for the top scorer on one occasion. It can be potentially life-threatening if severe and is particularly risky for the elderly or those with pre-existing conditions.