John Doukas, Latinized as Ducas () is the name of several Byzantine aristocrats and their descendants. It may refer to any of the following: John Doukas (Caesar) (), brother of Emperor Constantine X Doukas John Doukas (megas doux) ( – before 1136), brother-in-law of Alexios I Komnenos John Doukas (megas hetaireiarches) (), military commander and diplomat John Doukas (sebastokrator) (), cousin of Emperors Isaac II Angelos and Alexios III Angelos John Doukas Komnenos (1128–1176) John III Doukas Vatatzes (1193–1254), Emperor of Nicaea 1222–1254 John Komnenos Doukas (), ruler of Thessalonica and Thessaly 1237–1244 John Doukas (son of Michael II), son of Michael II of Epirus, hostage in Constantinople John I Doukas of Thessaly, ruler of Thessaly 1268–1289 John II Doukas of Thessaly, ruler of Thessaly 1303–1318 John II Orsini (also called Doukas), ruler of Epirus 1323–1335 John Uroš, ruler of Thessaly 1370–1373 John Ducas (investor) (born 1998), American entrepreneur