The Principality of Reuss-Gera (), officially called the Principality of Reuss Junior Line () after 1848, was a sovereign state in modern Germany, ruled by members of the House of Reuss. It was one of the successor states of the Imperial County of Reuss. The Counts Reuss, with their respective capitals and Residenzen at Gera, Schleiz, Lobenstein, Köstritz and Ebersdorf, were all elevated to the title of prince (Fürst) in 1806. Their successor branch heads shared that title, while their cadets were also each titled prince (Prinz).
When the German Empire collapsed at the end of World War I, the reigning Prince Reuss lost his crown along with all the other monarchs whose realms were within Germany. In 1927, Henrich XXVI's son, known as Count Heinrich Harry of Plauen (1890–1951), was adopted by his childless uncle, Prince Heinrich XXX (1864–1939), and the now-deposed dynasty agreed to accept him as "Prince Heinrich Harry Reuß", along with those of his male-line descendants born of unions complying with the family's 1902 rules that permitted marriages to countesses (Heinrich Harry's wife, Huberta von Tiele-Winckler was only a baroness in her own right, but belonged to a family of comital rank in Prussia).