The 88-year-old has a fever and is being treated with medication.
According to the Vatican, the Pope is in good spirits and has read some newspapers since he arrived at the hospital.
The Pope, who had part of one lung removed in his youth, is said to have had breathing difficulties in recent days and therefore been forced to ask officials to read his speeches.
Planned audiences on Saturday and Monday have been cancelled. Sunday's mass will be led by a cardinal, according to the Vatican.
In March 2023, Francis was hospitalized for bronchitis. In December last year, he was forced to cancel his trip to the climate conference in Dubai due to the same illness.
