Rainier was born in Monaco, the only son of Prince Pierre of Monaco, Duke of Valentinois and his wife, the Monegasque Hereditary Princess, Charlotte, Duchess of Valentinois. Rainier was the first native-born hereditary prince of Monaco since Honore IV in 1758. Rainier's mother was the only child of Prince Louis II of Monaco and Marie Juliette Louvet; she was later legitimized through formal adoption and subsequently named heiress-presumptive to the throne of Monaco. Rainier's father was a half-French, half-Mexican who adopted his wife's surname, Grimaldi, upon marriage and was made a prince of Monaco by Prince Louis, his father-in-law. Rainier had one sibling, Princess Antoinette, Baroness of Massy.[3]