Gölcük is a municipality and district in Kocaeli Province, Turkey. Its area is 217 square kilometres, and its population was 175,940 as of 2022.
The district lies on the southern shore of the Gulf of İzmit, the easternmost inlet of the Sea of Marmara. This position on the gulf gives Gölcük a sheltered coastal setting within the wider Marmara region of northwestern Turkey.
Note that the subject of this article is the Gölcük in Kocaeli Province. Other places in Turkey share the same name, including a town in the Balıkesir region, and they are distinct from this district.