31st hereditary ruler of the principality of Monaco (1949–2005). He was the son of Prince Pierre, count de Polignac, and Princess Charlotte de Monaco, daughter of Louis II, prince de Monaco. Rainier became a Grimaldi (i.e., received his mother's family name) in accord with a sovereign ordinance of March 18, 1920.

Rainier was educated in England and Switzerland and at the University of Montpellier in France. In 1944, during World War II, he served in the French army, and after the war he studied at the University of Paris. His mother having renounced her rights to succession in 1944, Rainier succeeded to the throne on May 5, 1949, just before Louis II's death. On April 19, 1956, he married Grace Kelly (Kelly, Grace), who became Princess Grace de Monaco (she died in an automobile accident in 1982); they had three children—Caroline, Albert (Albert II, prince de Monaco), and Stéphanie. Following Rainier's death in 2005, Albert assumed the throne.