Ostroh (Ukrainian: Острог, IPA: [oˈstrɔɦ]; Polish: Ostróg) is a city in Rivne Oblast, western Ukraine.

Since the 14th century, it was the seat of the powerful Ostrogski princely family, who developed their town into a great centre of learning and commerce.

[3] Ostróg slowly recovered, and in the second half of the 18th century, it became the site of a Jesuit college (see Collegium Nobilium).

In the interwar period, Ostróg belonged to the County of Zdołbunów, Volhynian Voivodeship of the Second Polish Republic.

On July 7, 1920, during the Polish–Soviet War, it was the site of a battle between a Polish unit under Wincenty Krajowski, and the Bolsheviks of Semyon Budyonny's 1st Cavalry Army.

Local mayor Stanisław Ludwik Żurakowski was arrested by the Soviets, then held in several prisons in Volhynia, and eventually deported to Kozelsk and murdered in the Katyn massacre in April 1940.

[5] The Nazi German occupation resulted in the establishment of the Reichskommissariat Ukraine (RKU), with headquarters in Rivne.