Gölcük is a municipality and district of Kocaeli Province in Turkey. Its area is 217 km² and its population was 175,940 in 2022. The district is located on the southern shore of the Gulf of İzmit, which is the easternmost inlet of the Sea of Marmara.
The settlement's position on the gulf gives it a direct frontage on the Sea of Marmara's easternmost waters, and it lies within Kocaeli Province, a district of northwestern Turkey. The administrative area covers both the town itself and the surrounding district, with a population recorded at 175,940 as of 2022.
Gölcük's defining geographic feature is its location on the Gulf of İzmit's southern shore, placing it on the Marmara coast of Turkey. The district's area of 217 km² encompasses this coastal position and its inland extent within Kocaeli Province.