Rainier III (Rainier Louis Henri Maxence Bertrand Grimaldi; 31 May 1923 – 6 April 2005) was Prince of Monaco from 1949 to his death in 2005. Rainier ruled the Principality of Monaco for almost 56 years.

Rainier was born at the Prince's Palace of Monaco, the only son of Hereditary Princess Charlotte and Prince Pierre. During his reign, he was responsible for the transformation of Monaco's economy, shifting from its traditional casino gambling base to its current status as a tax haven and cultural destination. The Prince also coordinated the substantial reforms of Monaco's constitution, which limited the powers of sovereign rule.

Rainier married American film star Grace Kelly in 1956, which generated global media attention. They had three children: Caroline, Albert and Stéphanie. Rainier died in April 2005 from complications relating to a lung infection compromised by years of cigarette smoking; he was succeeded by his son, Albert II.

Rainier was born on 31 May 1923 at Prince's Palace in Monaco, the first native-born prince since Honoré IV in 1758. Rainier's mother, Charlotte, was the only child of Louis II, Prince of Monaco, and his lover, Marie Juliette Louvet; she was legitimised through formal adoption and subsequently named heiress presumptive to the throne of Monaco. Rainier's father, Count Pierre of Polignac, who was half-French and half-Mexican, adopted his wife's dynasty, Grimaldi, upon marriage and was made a Prince of Monaco by his father-in-law. Rainier had an older sister, Princess Antoinette, Baroness of Massy. His parents divorced in 1933.

Rainier's early education was conducted in England, at the public schools of Summerfields in St Leonards-on-Sea, Sussex, and after 1935 at Stowe School, in Buckinghamshire. Rainier then attended the Institut Le Rosey in Rolle and Gstaad, Switzerland from 1939, before continuing to the University of Montpellier in France, where he obtained a Bachelor of Arts degree in 1943, before studying at Sciences Po Paris in Paris.

In 1944, the day before his 21st birthday, Rainier's mother renounced her right to the Monegasque throne and Rainier became Prince Louis's direct heir. In World War II, Rainier joined the Free French Army in September 1944, and served under General Joseph de Goislard de Monsabert as a second lieutenant. As a soldier, he witnessed action during the German counter-offensive in Alsace. Rainier received the French Croix de Guerre with bronze star (representing a brigade level citation) and was given the rank of Legion of Honor in 1947. Following his decommission from the French Army, he was promoted by the French government to captain in April 1949 and colonel in December 1954.

Rainier became the Sovereign Prince of Monaco at the age of 25 upon the death of his maternal grandfather, Prince Louis II, on 9 May 1949.

After ascending the throne, Rainier III worked to recoup Monaco's lustre, which had become tarnished through financial neglect and scandal (his mother, Princess Charlotte, took a noted jewel thief known as René the Cane as her lover). Upon ascension, the Prince found a treasury that was practically empty. Monaco's traditional gambling clientele, largely European aristocrats, found themselves with reduced funds after World War II. Other successful gambling centres had opened to compete with Monaco. To compensate for the loss of income, Rainier decided to promote Monaco as a tax haven, commercial centre, real-estate development opportunity, and international tourist attraction. The early years of his reign saw the overweening involvement of the Greek shipping tycoon Aristotle Onassis, who took control of the Société des Bains de Mer and envisioned Monaco as solely a gambling resort. Prince Rainier regained control of SBM in 1964, effectively ensuring that his vision of Monaco would be implemented.