Palace officials said the 81-year-old had been improving for several days after being hospitalized March 7, but late Tuesday suddenly declined. He has battled heart and lung problems for the several years.
His son, Crown Prince Albert, 47, and daughter Caroline, 48, rushed back to the tiny principality Tuesday to be with their father. His youngest daughter, Stephanie, 40 lives in Monte Carlo and has visited her father repeatedly in the past two weeks.
Rainier has governed the wealthy state on the French Riviera since 1949, and is Europe's longest-reigning monarch. His wife, the late Hollywood star and Oscar winner Grace Kelly, died in a car crash in 1982.