Charles III (Charles Honoré Grimaldi; 8 December 1818 – 10 September 1889) was Prince of Monaco and Duke of Valentinois from 20 June 1856 to his death. He was the founder of the famous casino in Monte Carlo, as his name in Monegasque and Italian was Carlo III. He was born in Paris, the only son of Florestan, Prince of Monaco, and Maria Caroline Gibert de Lametz.

While he was Hereditary Prince, Charles was married on 28 September 1846 in Brussels to Countess Antoinette de Mérode-Westerloo.

He succeeded his father Prince Florestan in 1856.

During his reign, the towns of Menton and Roquebrune, constituting some 80 percent of Monegasque territory, were formally ceded to France, paving the way for formal French recognition of Monaco's independence. Rebellions in these towns, aided by the Kingdom of Sardinia, had exhausted Monaco's military resources for decades.

The Principality was in dire need of cash flow, so Prince Charles and his mother, Princess Caroline, had the idea of erecting a casino. The Monte Carlo Casino was designed, according to the Prince's liking, in the German style and placed at the site of Les Spélugues. Monte Carlo (in English, Mount Charles) itself takes its name from Charles, after all its founder. Charles established a society (business) to run the Casino; this society is today the Société des bains de mer de Monaco.

Under Charles III, the Principality of Monaco increased its diplomatic activities; for example, in 1864, Charles III concluded a Treaty of Friendship with the Bey of Tunis, Muhammad III as-Sadiq, which also regulated trade and maritime issues.

Monte Carlo is named after Charles III. It stands for the "Mount Charles" in Italian.

The Order of Saint-Charles was instituted on 15 March 1858, during the reign of Prince Charles III.