John III Doukas Vatatzes

John III Doukas Vatatzes (c. 1192–1254) was probably the son of the general Basil Vatatzes. Theodore I Laskaris married him to his daughter Irene, and he took the Nicaean throne in 1221.

He defeated rival claimants at Poimanenon in 1224, seizing Adrianople, but lost it in 1227. He besieged Latin Constantinople in 1235–1236, imposed his suzerainty on Thessalonica in 1242, and annexed it and much of Thrace in 1246.

A collector of manuscripts, he died at Nymphaion in 1254 and was later venerated as Saint John the Merciful. His son Theodore II succeeded him.