| Heinrich XX | |
| Prince Reuss of Greiz | |
| Spouse: | Princess Sophie of Löwenstein-Wertheim-Rosenberg Princess Caroline of Hesse-Homburg |
| Issue: | Christiane, Princess Hugo of Schönburg-Waldenburg Prince Heinrich XXI Heinrich XXII, Prince Reuss of Greiz Prince Heinrich XXIII Marie, Countess Friedrich of Ysenburg and Büdingen in Meerholz |
| House: | House Reuss of Greiz |
| Father: | Heinrich XIII, Prince Reuss of Greiz |
| Mother: | Princess Wilhelmine Louise of Nassau-Weilburg |
| Birth Date: | 29 June 1794 |
| Birth Place: | Offenbach, Landgraviate of Hesse-Darmstadt |
| Death Place: | Greiz, Principality of Reuss-Greiz |
Heinrich XX, Prince Reuss of Greiz (German: Heinrich XX Fürst Reuß zu Greiz; 29 June 17948 November 1859) was Prince Reuss of Greiz from 1836 to 1859.
Heinrich XX was born at Offenbach, Landgraviate of Hesse-Darmstadt, younger surviving son of Heinrich XIII, Prince Reuss of Greiz (1747–1817), (son of Heinrich XI, Prince Reuss of Greiz and Countess Conradine Reuss of Köstritz) and his wife,Princess Wilhelmine Louise of Nassau-Weilburg (1765–1837), (daughter of Charles Christian, Prince of Nassau-Weilburg and Princess Carolina of Orange-Nassau).
At the death of his elder brother on 31 October 1836, Heinrich XX succeeded as the Prince Reuss of Greiz because of the Salic law that applied in the German principalities, his brother had died with no male heir.
Heinrich XX kept the principality administration based on absolutist principles at least until 1848 when because of the Revolution, was forced to issue a constitution but never came into force. The Prince, however, distinguished himself in military service in favor of the Austrian Empire.
Heinrich XX married on 25 November 1834 at Castle Haid in Haid to Princess Sophie of Löwenstein-Wertheim-Rosenberg (1809–1838), third daughter of Karl, Prince of Löwenstein-Wertheim-Rosenberg, and his wife, Countess Sophie Luise of Windisch-Graetz . They had no children.
He married secondly on 1 October 1839 in Homburg vor der Höhe to Princess Caroline Amalie of Hesse-Homburg, eldest daughter of Gustav, Landgrave of Hesse-Homburg, and his wife, Princess Louise of Anhalt-Dessau.
They had five children: