The head of the tiny monarchy on the Mediterranean coast since 1949, Rainier had endured heart and respiratory problems for 10 years before being hospitalized March 7, the BBC reported.
During the days before his death, his heart, lungs and kidneys had begun to fail and he had been placed on a respirator.
Ranier had three children with his late wife, U.S. movie star Grace Kelly -- Crown Prince Albert, 47; Princess Caroline, 48; and Princess Stephanie, 40 -- all of whom had been at his bedside for several days prior to the monarch's death.
Rainier's marriage in 1956 to the stunning and elegant Kelly seemed to define a fairy tale blend of charm and romance. She died in a car crash in 1982.
The prince's stewardship of the 700-year-old Grimaldi dynasty has been marked by a declining reliance on tourism and gambling and development of a vigorous business sector.
In recent years, he delegated more duties to his son and heir, Prince Albert.