Byzantine humanist scholar, philosopher, and theologian whose 37-volume Byzantine History, a work of erudition, constitutes a primary documentary source for the 14th century.

Having gained the favour of the emperor Andronicus II Palaeologus (1282–1328) and of ecclesiastics in Constantinople, Gregoras was entrusted with diplomatic missions, including a legation to the Serbian king Stephan Uroš III in 1326. With the downfall of his patrons, however, Gregoras was, as was the custom, forced to retire to a nearby monastery. Gregoras emerged victorious in a philosophical disputation, accompanied by polemical tracts, against the monk Barlaam of Calabria, an outspoken Aristotelian scholastic, and was recognized as Constantinople's leading academician. A theological controversy with deep political ramifications followed, in which Gregoras contended with the doctrine of Hesychasm. After the accession of the emperor John VI Cantacuzenus (1347), the Hesychast party, led by the monks of Mount Athos (Athos, Mount), enjoyed preference, requiring Gregoras to retire from public life. In 1351 he was excommunicated by a local church council, and after his death about 1360 his body was dragged through the streets of Constantinople.

His most renowned work, the Byzantine History, chronicles the events of the Eastern Empire from the time of the Latin conquest in the Fourth Crusade (1204) to 1359. Supplementing the work of the earlier 14th-century historian George Pachymeres (Pachymeres, George), Gregoras enlarged on the philosophical and theological disputes in which he had engaged. His Correspondence, containing more than 160 letters, is a rich source for knowledge of the outstanding Byzantine ecclesiastical and political figures of the period. Among Gregoras's other notable works are philosophical dialogues against the Sophists (Sophist), studies in astronomy, a commentary on the Almagest of the 2nd-century astronomer Ptolemy, eulogies for several emperors, and a proposal for calendar reform that anticipated Pope Gregory XIII's revision of 1582.