Hagia Sophia in İznik (Nicaea) in Bursa Province, Turkey, was built as a Byzantine-era basilican church. Converted into the Orhan Mosque after the Ottoman conquest, it was turned into a museum in 1935. The church is now once again in service as a mosque. It is in the town centre of İznik, within the old walled area.

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.