Prince Joachim Franz Humbert of Prussia (17 December 1890 – 18 July 1920) was the youngest son and sixth child of Wilhelm II, German Emperor, by his first wife, Augusta Victoria of Schleswig-Holstein.

His great-grandson is Grand Duke George Mikhailovich, the heir apparent to Maria Vladimirovna, a claimant to the disputed Headship of the Imperial Family of Russia.

Prince Joachim spent his childhood with his siblings at the New Palace in Potsdam, and his school days at the Prinzenhaus in Plön, in his mother's ancestral Schleswig-Holstein, as his brothers had been before him.

The couple had one son, Prince Karl Franz Josef Wilhelm Friedrich Eduard Paul (15 December 1916 in Potsdam – 22 January 1975 in Arica, Chile).

During the Easter Rising in Dublin in 1916, some republican leaders, including Patrick Pearse and Joseph Plunkett, contemplated giving the throne of an independent Ireland to Prince Joachim.

[2][3] Pearse and Plunkett thought that if the rising were successful and Germany won the First World War, an independent Ireland would be a monarchy with a German prince as king, like Romania and Bulgaria before it.

"[6]Ernest Blythe recalls that in January 1915 he heard Plunkett and Thomas MacDonagh express support for the idea at an Irish Volunteers meeting.

Following the German Revolution in November 1918, the Emperor was forced to abdicate, thus depriving Joachim of his titles, position and prospects for heading any newly established monarchies in Europe.

"[12] Although the Kaiser initially tried to convince his wife that the death of their youngest son had simply been an accident, she immediately realised what had happened, and interrupted him with the remark: ""He has shot himself!"

On 5 October 1940, Prince Karl married Princess Henriette Hermine Wanda Ida Luise von Schönaich-Carolath (25 November 1918 – 16 March 1972).