Kostopil is a small city on the Zamchysko river in Rivne Oblast, western Ukraine. It was the administrative center of the Kostopil Raion up to 2…Kostopil is a small city on the Zamchysko river in Rivne Oblast, western Ukraine. It was the administrative center of the Kostopil Raion up to 2020, but is now within the Rivne Raion. Population: 30,838.