Виллем II
Heinrich I was born at Krietern, Breslau, Kingdom of Prussia, eldest child of Prince Heinrich XXXIV Reuss of Köstritz (1887–1956), (son of Prince Heinrich XXVIII Reuss of Köstritz, and Countess Magdalene of Solms-Laubach) and his wife, Princess Sophie Renata Reuss of Köstritz (1884–1968), (daughter of Prince Heinrich VII Reuss of Köstritz and Princess Marie of Saxe-Weimar-Eisenach). Through his mother he was descendant of King William II of the Netherlands