Princess Antoinette of Monaco, Baroness of Massy (Antoinette Louise Alberte Suzanne Grimaldi; 28 December 1920 – 18 March 2011) was a member of the princely family of Monaco and the elder sister of Prince Rainier III and aunt of Albert II, Prince of Monaco. Her parents were Count Pierre de Polignac and Princess Charlotte, Duchess of Valentinois.

She was born in Paris of French and Mexican ancestry.

Princess Antoinette had a long-term liaison with Alexandre-Athenase Noghès, a Monegasque-born attorney and international tennis champion, in the mid-1940s. The couple had three children born out-of-wedlock who were legitimated by their parents' eventual marriage and, henceforth, included in the line of succession to the Monegasque Throne until the death of Antoinette's brother, Prince Rainier III, in 2005; Elisabeth-Anne de Massy (born 1947), Christian Louis de Massy (Noghès) (born 1949), and Christine de Massy (Noghès) (1951-1989).

Having divorced Noghès, she and her lover Jean-Charles Rey hatched a plan to depose her brother Rainier III, Prince of Monaco and declare herself regent on the basis of having a son who would one day inherit the throne. She circulated rumours that Rainier's fiancee, actress Gisèle Pascal, was infertile. This led to the breakup of the relationship.

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