Kadıköy is a municipality and district on the Asian side of Istanbul Province, Turkey. Its area is 25 km2, and its population is 467,919 (2023). It is a large and populous area in the Asian side of Istanbul, on the northern shore of the Sea of Marmara. It partially faces the historic city centre of Fatih on the European side of the Bosporus. It is bordered by the districts of Üsküdar, to the northwest, Ataşehir, to the northeast, and Maltepe, to the southeast.

The Sea of Marmara, also known as the Sea of Marmora or the Marmara Sea, is a small inland sea located entirely within the borders of Turkey. It connects the Black Sea to the Aegean Sea via the Bosporus and Dardanelles straits, separating Turkey’s European and Asian sides. It has an area of 11,350 km2 (4,380 sq mi), and its dimensions are 280 km × 80 km. Its greatest depth is 1,370 m (4,490 ft).