Prince Sigismund of Prussia (d. 1866)
Prince Francis Frederick Sigismund of Prussia (German: Franz Friedrich Sigismund; 15 September 1864 – 18 June 1866) was the fourth child and third son of Crown Prince Frederick William of Prussia (later King of Prussia and German Emperor as Frederick III), and Victoria, Princess Royal, eldest daughter of the British Queen Victoria.

He was born at the New Palace in Potsdam, Germany, in 1864 and known as "Sigi" to his family. He died from meningitis at the New Palace on 18 June 1866, aged twenty-one months. He was buried in the royal mausoleum of the Friedenskirche at Potsdam. His mother's grief and despair were intense as his father, leading the Prussian army into battle against Austria, had taken all available doctors thus making it impossible for her to alleviate the suffering of her child or prevent his death.

Prince Sigismund was the first grandchild of Queen Victoria to die, almost 115 years before his last cousin Princess Alice, Countess of Athlone, who died in 1981.