"Friends, I had a very strong flu in Ibiza and I had to be hospitalised on Friday but everything is in order", the legendary Brazil forward said on Twitter.
The former footballer, Ronaldo Nazário de Lima, was admitted into the Hospital de Can Misses in Ibiza last Friday.
It is reported he was transferred to a private hospital called Clinica Nuestra Señora del Rosario later the same day.
The paper quoted sources from the hospital who said he was still in intensive care but making good progress.
"Tomorrow I will be allowed to leave the hospital and I will return home".
Contacted by AFP, a hospital spokeswoman said she was unable to give any information for privacy reasons. "Thanks for all your support and your messages".
Nobody from the private hospital Ronaldo is now being treated at could be reached for comment, according to the Telegraph.
However the pneumonia is likely to have been triggered by an unrelated bacterial infection.
Ronaldo owns property on Ibiza, a Spanish Mediterranean island, and is a frequent visitor.
Reports in Spain claim the former striker owns a house in Ibiza and is understood to have flown out for a summer break only a few days ago. He represented his country in three World Cups, including the 2002 edition, when Brazil won its fifth tournament title.