After the death of Alexander, winning the fight with Antigonus, Lysimakhos named the city
Nikaia, which was the name of Antipatros' daughter. The city was united with Bithynia Kingdomin B.C. 293 and was decorated with important architectural buildings. Nikaia had been the capital of Bithynia Kingdom for some time, and later kept on being an important settlement in Rome.
In early summer of year 325, the first consulate which is essential in Christianity was gathered in İznik. After this meeting where emperor Constantinus also took part, holidays of christianity and so called Nikaia Laws (a text consisting of twenty articles) was accepted. In year 787, 7th consulate was gathered in Hagia Sophia of İznik. Moreover, at the end of 6th crusade, when Byzantine lost İstanbul, Latin Empire of İznik was founded by Byzantinian dynasty. That empire later on conqured İstanbul and rebuilt Byzantine empire.
İznik had been the capital of Seljuqs and Ottomans as well. İznik was conquered by Ottoman
army in 1331. In Ottoman time, İznik had become a center of art, trade and culture. Osman Gazi had thought many well-known lectures in his madrasa. Sufis like Davud-u Kayseri, Ebul Fadıl Musa, Eşrefoğlu Abdullah Rumi had lived in İznik and has produced some of their works there.
The first mosque, madrasa and imaret of Ottoman time was constructed in İznik.
In 14th, 15th and 16th centuries, İznik had been a center of art. World-wide known chinawares and ceramics were crafted there.