Ostroh is a city in Rivne Oblast, western Ukraine. It is situated on the Horyn River. Ostroh was the administrative center of Ostroh Raion until 2020, but as a city of oblast significance did not belong to the raion. Currently the city is the centre of Ostroh urban hromada. Population: 14,894.

Rivne Oblast, also referred to as Rivnenshchyna, is an oblast (province) in western Ukraine. Its administrative center is Rivne. The surface area of the region is 20,100 square kilometres (7,800 sq mi). Its population is: 1,141,784.