The Cellamare conspiracy of 1718 ((French) Conspiration de Cellamare) was a conspiracy against the then Regent of France, Philippe d'Orléans (1674–1723). Concocted in Spain, the plot was the brainchild of Antonio del Giudice, Prince of Cellamare.
Antonio del Giudice was named the Spanish Ambassador to the French Court in 1715 during the reign of Louis XIV of France; Louis XIV died in September that year and his successor, Louis XV of France, aged five, was put under the care of the Duke of Orléans who was a nephew (as well as son-in-law) of the late Louis XIV. Antonio del Giudice went to France during the Regency of Philippe d'Orléans.
At the instigation of Guillaume Dubois, Secretary of State for Foreign Affairs, France formed the Quadruple Alliance between England, Holland and the Holy Roman Empire in order to combat the possible personal union between France and Spain, should the five-year-old Louis XV die; Spain was then ruled by Louis XV's uncle, Phillip V, who had been king of Spain since the death of Charles II in 1700. The War of the Spanish Succession had resulted in the Treaty of Utrecht of April 1713; that treaty underlined that neither Philip nor his descendants could inherit the French throne, he having agreed to this previously.
The Prince of Cellamare and his embassy wanted to try to depose Philippe d'Orléans from power and make Philip V the Regent for his nephew. The plan was greatly supported by some of Philippe d'Orléans most notorious enemies, namely the duke and duchess du Maine, Louis Auguste de Bourbon and his wife, Louise Bénédicte de Bourbon, who had entered into correspondence with the Spanish Prime Minister Giulio Alberoni - a favourite of Queen Elisabeth Farnese.
With the support of the Spanish ambassador, a plot to carry out the transferral was soon woven into the entourage of the duchess. According to the memoirs of the duchess' lady-in-waiting the Baroness de Staal, Melchior de Polignac and the Duke of Richelieu, among others, were part of the plot.
The Duke of Richelieu was at the time having a romantic affair with one of the Regent's daughters, Charlotte Aglaé d'Orléans. The Baroness's memoirs said that the duke and duchess of Maine had wanted to call a meeting between the États généraux regarding the matter.
The correspondence between the duchess of Maine and Alberoni was later intercepted by the police and thus reported to the Regent who acted swiftly; on 9 December, the Prince of Cellamare was arrested and sent back to Spain; Alberoni was arrested on 5 December 1718 at Poitiers; the duchess was exiled to Dijon while her husband was imprisoned in the fortress of Doullens in Picardy. The Duke of Richelieu was imprisoned in the Bastille where he was later visited by his lover Charlotte Aglaé d'Orléans dressed in disguise.
Despite this, the guilty members of the plot were all pardoned by 1720 and allowed to return to their residence.
On 9 January 1719, France declared war on Spain, following in the footsteps of England who had done so a couple of weeks earlier, on 27 December 1718.
Two years later, the aims of the Cellamare conspiracy were revived in the Pontcallec Conspiracy, four leaders of which were executed.