| Conventional Long Name: | Principality of Reuss-Greiz Principality of the Reuss Elder Line |
| Common Name: | Reuss-Greiz |
| Government Type: | Principality |
| Year Start: | 1778 |
| Year End: | 1918 |
| Date Start: | 12 May |
| Date End: | 11 November |
| P1: | Imperial County of Reuss |
| Flag P1: | Flag of Reuss-Lobenstein.svg |
| S1: | People's State of Reuss |
| Flag S1: | Flagge Fürstentum Reuß ältere Linie.svg |
| Image Map Caption: | Reuss-Greiz within the German Empire |
| Image Map2: | Shield Reuss OL.png |
| Image Map2 Caption: | Reuss-Greiz within Thuringia |
| Capital: | Greiz |
| National Motto: | Ich Bau Auf Gott "I build on God" |
| National Anthem: | Gott, erhalt' in Deiner Gnaden Unsern Fürsten "God, Save in Your Mercy Our Prince" |
| Title Leader: | Prince |
| Leader1: | Heinrich XI |
| Year Leader1: | 1778–1800 |
| Leader2: | Heinrich XIII |
| Year Leader2: | 1800–1817 |
| Leader3: | Heinrich XIX |
| Year Leader3: | 1817–1836 |
| Leader4: | Heinrich XX |
| Year Leader4: | 1836–1859 |
| Leader5: | Heinrich XXII |
| Year Leader5: | 1859–1902 |
| Leader6: | Heinrich XXIV |
| Year Leader6: | 1902–1918 |
| Title Deputy: | Minister of State |
| Deputy1: | Franz von Grün |
| Year Deputy1: | 1782–1833 |
| Deputy2: | Ernst von Meding |
| Year Deputy2: | 1901–1918 |
| Today: | Germany |
The Principality of Reuss-Greiz (German: Fürstentum Reuß-Greiz), officially called the Principality of the Reuss Elder Line (German: Fürstentum Reuß älterer Linie) after 1848, was a sovereign state in the German Empire, ruled by members of the House of Reuss. The Counts Reuss of Greiz, Lower-Greiz and Upper-Greiz (German: Reuß zu Greiz, Untergreiz und Obergreiz) were elevated to princely status in 1778 and thereafter bore the title of Prince Reuss, Elder Line, or Prince Reuss of Greiz.
Similarly to the more numerous Reuss Junior Line, the male members of this house were all named "Heinrich", in honour of Emperor Heinrich VI, who had benefited the family. They were numbered sequentially by birth, rather than by reign, with the last series beginning with Heinrich I (born 1693) and ending with Heinrich XXIV (1878–1927).
The territory had an area of 317 km2 and over 72,000 inhabitants in 1910.
Reuss-Greiz preserved the Frankfurt Parliament flag, which later became the flag of Germany.
In 1919, in the aftermath of World War I, the territory of the Elder Line was merged with that of the Junior Line as the People's State of Reuss, which was incorporated into the new state of Thuringia in 1920. The Elder Line died out in 1927 with the death of the childless Heinrich XXIV, after which its claims were passed to the Junior Line.