Ivan Asen II of Bulgaria
Konstantin divorced his first wife (whose name is unknown), and married Irene Doukaina Laskarina in 1258. Irene was the daughter of Theodore II Laskaris, Emperor of Nicaea, and Elena of Bulgaria, a daughter of Ivan Asen II of Bulgaria. The marriage with a descendant of the Bulgarian royal family strengthened his position. He was thereafter called Konstantin Asen. The marriage also forged an alliance between Bulgaria and Nicaea, which was confirmed one or two years later, when the Byzantine historian and official George Akropolites came to Tarnovo.