The 41-year-old Brazilian is said to have been diagnosed with the illness, after being rushed to Can Misses Hospital on the island.
The former Barcelona, Real Madrid and Inter Milan player then asked to be transferred to a private hospital later on Friday evening to continue to receive treatment, according to Ibizan daily newspaper Diario de Ibiza.
Ronaldo is a frequent visitor to the Balearic party island of Ibiza and owns a property there.
Ronaldo is believed to have arrived in Ibiza for a summer holiday a few days ago before he fell sick.
At the time, the Brazilian managed to handle the indisposition humorously with his trademark smile via tweet from his hospital bed saying: "Good morning to all those who began the New Year with dengue".
After hitting 62 goals in 98 appearances for his country, as well as being a prolific scorer for some of the top clubs in Europe, Ronaldo is widely regarded as one of the best players in the history of the game.
Pneumonia is swelling of the tissue in one or both lungs and is usually caused by a bacterial infection.
During his playing career, Ronaldo was famous for his blistering pace and phenomenal goal-scoring record.
Symptoms - which include a cough and difficulty breathing - can develop suddenly over 24 to 48 hours.
Since retirement, however, he has battled thyroid problems and gained significant weight. It can be potentially life-threatening if severe and is particularly risky for the elderly or those with pre-existing conditions.