John III Megas Komnenos or Grand Comnenus (Greek: Ἰωάννης Μέγας Κομνηνός, Iōánnēs Mégas Komnēnós; c. 1321 – 1362) was emperor of Trebizond from September 4, 1342, to May 3, 1344.

However, in 1342 the leaders of the Scholarioi, Niketas Scholares and Gregory Meitzomates, visited him there and persuaded the young man to come with them to Trebizond and take the throne.

With the approval of the Byzantine government, the group set out for Trebizond in September 1342 after enlisting the support of three Genoese galleys, bringing their little fleet to a total of five ships.

After a short but fierce fight John and his supporters captured the city on September 4, aided by a popular uprising in their favor.

[2] Upon the death of the megas doux, Niketas marched to Limnia where he freed Michael then returned with him to Trebizond to depose John.