The 41-year-old Brazilian was rushed to Can Misses Hospital on Friday after being diagnosed with the illness.
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.
This story will be updated when more information on Ronaldo's condition is known.
Considered one of the best strikers of his generation, Ronaldo played for a host of top European clubs including Real Madrid, Inter Milan and Barcelona.
The striker retired from the game in 2011 and reportedly owns a house in Ibiza and is a regular visitor to the island.
Ronaldo is believed to have arrived in Ibiza for a summer break a few days ago.
Pneumonia affects the lungs and is caused by bacterial infection.
Since retirement, however, he has battled thyroid problems and gained significant weight.
Symptoms - which include a cough and difficulty breathing - can develop suddenly over 24 to 48 hours. It can be potentially life-threatening if severe and is particularly unsafe for the elderly or those with pre-existing conditions. Ronaldo scored twice in the game and was named man of the match as his side picked up the trophy for the fifth time. He won the Golden Ball at the 1998 World Cup and the Golden Boot four tears later as the Selecao triumphed.