Prince Rainier III (Rainier Louis Henri Maxence Bertrand Grimaldi) (born May 31, 1923), is the hereditary Prince and head of state of the Principality of Monaco. His full titles and styles are His Serene Highness Rainier III, Prince of Monaco, Duc de Valentinois, Marquis des Baux, Comte de Carlades, Baron de Buis, Seigneur de Saint-Rémy, Sire de Matignon, Comte de Thorigny, Baron de Saint-Lô, de la Luthumière et de Hambye, Duc d'Estouteville, de Mazarin et de Mayenne, Prince de Château-Porcien, Comte de Ferrette, de Belfort, de Thann et de Rosemont, Baron d'Altkirch, Seigneur d'Isenheim, Marquis de Chilly, Comte de Longjumeau, Baron de Massy, Marquis de Guiscard. Prince Rainier is styled "His Serene Highness".

He was born in the principality of Monaco, the only son of Princess Charlotte Louise Juliette of Monaco (1898 - 1977) – the illegitimate and later legitimated Algeria-born daughter of Louis II of Monaco and Marie Juliette Louvet, reportedly either a cabaret singer or a laundress – and her husband, Prince Pierre-Marie-Xavier-Antoine-Melchior of Monaco (1895 - 1964), né Count Pierre de Polignac. Rainier III had one sister, Princess Antoinette.

Rainier was first sent to study at St. Leonards-on-Sea in England, then at Stowe, a prestigious private English school. From there, he went to Institute Le Rosey, Rolle/Gstaad, Switzerland before going on to university in France.

His grandfather, Prince Louis II, had been a General in the French army and during World War II. Rainier served as an artillery officer in the army. Second Lieutenant Rainier fought so courageously during the German counter-offensive in Alsace that he won the Croix de Guerre, the Bronze Star and was given the rank of Chevalier in the Legion of Honor.

On May 9, 1949, Rainier became the Prince of Monaco on the death of his grandfather, Louis II. His mother had previously renounced her rights to the throne in his favor.

In 1956, His Serene Highness the Prince Rainier III of Monaco married Oscar-winning American actress Grace Patricia Kelly (1929 - 1982). Their children are:

1. Princess Caroline Louise Marguerite, born January 23, 1957
2. Prince Albert Alexandre Louis Pierre, heir to the throne, and Marquis of Baux, born March 14, 1958
3. Princess Stephanie Marie Elisabeth, born February 1, 1965

As Prince, Rainier III was responsible for Monaco's new constitution in 1962 that significantly reduced the power of the sovereign. The changes ended autocratic rule, placing power with the prince and a National Council of eighteen elected members.

He is the world's second longest-serving Head of State, ranking just below King Rama IX of Thailand but above Queen Elizabeth II of the United Kingdom.

In the years since his wife's death, he was reportedly involved with a distant cousin, Princess Ira von Fürstenberg, a former movie actress turned jewelry designer who is also a Fiat heiress and the former sister-in-law of fashion designer Diane von Furstenberg. Like him, she is a great-grandchild of Lady Mary Douglas-Hamilton, the Scottish-Bavarian wife of Prince Albert I of Monaco, though by Lady Mary's second marriage.

Rainer III is also known to be a football fan, having followed AS Monaco FC to the 2004 Champions League final.

Early in 2004 he was hospitalized for coronary problems. On October 26, 2004, he was checked into a hospital for a lung infection.