There had been concerns for the health of the 41-year-old, who retired in 2011, after reports in Spain that he had been diagnosed with pneumonia and was in intensive care at the island’s Can Misses hospital.
The former Barcelona forward provided a medical update on social media on Sunday afternoon, saying he went into hospital on Friday suffering from a bad bout of flu.
He tweeted: “Friends, I had a strong flu picture in Ibiza and I had to be boarded on Friday but it’s all in order.
“I get the discharge (on Monday) and come back home. Thank you all for your love and your messages!”
Ronaldo, whose club career took in spells at Cruzeiro, PSV Eindhoven, Barcelona, Inter Milan, Real Madrid, AC Milan and Corinthians, is on holiday in Ibiza, where he owns a house.
The striker’s career ended seven years ago when he retired after two seasons with Corinthians.
He scored 62 goals in 98 games for Brazil, as well as being a prolific scorer for some of Europe’s elite clubs.