Gregory Callimachi (Greek: Γρηγόριος Καλλιμάχης, Romanian: Grigore Callimachi; 1735 – 9 September 1769) was a Phanariote who served as Prince of Moldavia from 1761 to 1764, and 1767 to 1769.

Replaced by Grigore III Ghica in March 1764, he returned to his throne in January 1767.

It was the Russo-Turkish War of 1768-1774 that led to his death: accused of collusion with the Russian Empire, he was deposed in June 1769 and sent to Istanbul where he was tried and sentenced to death for treason with the Grand Dragoman Nicolas Soutzo and the Grand Vizier Yağlıkçızade Mehmed Emin Pasha.

In September 1769, Callimachi is tied up and his severed head is exposed to the Bâb-ı Hümâyûn, that is to say to the outer door of the Topkapı Palace, with a yafta (explanatory notice) specifying his crimes.

Gregory Callimachi married Helena Mavrocordatos, from whom he had two children: