İznik is a municipality and district of Bursa Province, Turkey. Its area is 753 km2, and its population is 44,236 (2022). The town is at the site of the ancient Greek city of Nicaea, from which the modern name derives. The town lies in a fertile basin at the eastern end of Lake İznik, with ranges of hills to the north and south. As the crow flies, the town is only 90 kilometres southeast of Istanbul but by road it is 200 km around the Gulf of İzmit. It is 80 km by road from Bursa.

Nicaea, also known as Nikaia, was an ancient Greek city in the north-western Anatolian region of Bithynia that is primarily known as the site of the First and Second Councils of Nicaea, the Nicene Creed, and as the capital city of the Empire of Nicaea following the Fourth Crusade in 1204, until the recapture of Constantinople by the Byzantines in 1261. Nicaea was also the capital of the Ottomans from 1331 to 1335.