Kocaeli Province is a province and metropolitan municipality of Turkey and one of only three not to have the same official name as its capital, İzmit, which is thus also sometimes called Kocaeli. Its area is 3,397 km2 (1,312 sq mi), and its population is 2,102,907 (2023). The province is the successor of the Ottoman-era Sanjak of Kocaeli. The largest towns in the province are İzmit and Gebze. The traffic code is 41.