Theodora Kantakouzene (Greek: Θεοδώρα Καντακουζηνή; died after 1381) was a Byzantine princess, the daughter of Emperor John VI Kantakouzenos and the fifth wife of the Ottoman Sultan Orhan Gazi.

After her marriage to Orhan, she became known among the Turks as "Maria Hatun" (Turkish for "Lady Maria").

Theodora was one of the three daughters of Emperor John VI Kantakouzenos by his wife Irene Asanina. The historian Nikephoros Gregoras erroneously calls her "Maria" in one passage. In January 1346, to cement her father's alliance with the rising Ottoman emirate and to prevent the Ottomans from giving their aid to the Empress-regent Anna of Savoy during the ongoing civil war, she was betrothed to the Ottoman ruler, Orhan Gazi. The marriage took place in the summer of the same year. Her parents and sisters escorted her to Selymbria, where Orhan's representatives, including grandees of his court and a cavalry regiment, arrived on a fleet of 30 ships. A ceremony was held at Selymbria, where Orhan's envoys received her and escorted her to the Ottoman lands in Bithynia, across the Marmara Sea, where the actual wedding took place.

Theodora remained a Christian after her marriage, and was active in supporting the Christians living under Ottoman rule. In 1347 she gave birth to her only son, Şehzade Halil, who was captured by Genoese pirates for ransom while still only a child. The Byzantine emperor John V Palaiologos was instrumental in his eventual release. Later, Halil married Irene, a daughter of John V Palaiologos and Theodora's sister, Helena Kantakouzene.

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