The 80-year-old ruler was admitted to the Principality's cardio-thoracic unit Saturday night, his third trip to the hospital in recent months, Riviera Radio reported Monday.
He was hospitalized with the flu before Christmas and was treated for fatigue early in the New Year.
The prince, who has a history of heart problems, has not been seen in public since Dec. 16, 2003.
Prince Rainier III is one of the world's longest-reigning monarchs. His wife, the former Hollywood actress Grace Kelly, died in a car crash in 1982.
Notable deaths of 2014 [PHOTOS]