Mykolayiv (mēˌkəläˈyĭf) [key], Rus. Nikolayev, city (1991 est. pop. 511,000), capital of Mykolayiv region, S Ukraine, at the confluence of the Buh and Inhul rivers and on the bank of the Dnieper-Buh estuary. A major seaport and rail junction, Mykolayiv exports grain, iron, and manganese. It has shipyards, machinery plants, and an aluminum smelter. Founded in 1784 as a fortress near the site of the ancient Greek colony of Olbia, the city was named Mykolayiv in 1788 when it became a shipbuilding center.