The Principality of Reuss-Greiz, was a state in Germany, ruled by members of the House of Reuss. The Counts Reuss of Greiz, Lower- and Upper Greiz, were elevated to princely status in 1778. Its members bore the title Prince Reuss, Elder Line, or Prince Reuss of Greiz. Similarly to the more numerous Reuss Younger Line, the male members of this house were all named "Heinrich", in honour of Heinrich VI who had benefited the family, and were numbered sequentially by birth, rather than by reign, the last series beginning with Heinrich I (born 1693) and ending with Heinrich XXIV (1878–1927).
The Principality of Reuss Elder Line had an area of 317 km² and a population of 71,000 (1905). Its capital was Greiz.
In 1919, in the aftermath of the Great War, the territory of the Elder Line was merged with that of the Junior Line as the Republic of Reuss, which later that year was incorporated into the new state of Thuringia. The Reuss Elder Line died out with the death of the childless Heinrich XXIV in 1927, after which its claims were passed to the younger line.