Ostroh (Ukrainian: Остро́г; Russian: Остро́г, Ostrog, Polish: Ostróg) is a historic city located in Rivne Oblast (province) of western Ukraine, located on the Horyn River. Ostroh is the administrative center of the Ostroh Raion (district) and is itself designated as a special administrative subordination within the oblast. The current estimated population is around 14,801 (as of 2001).

The Hypatian Codex first mentions Ostroh in 1100, as a fortress of the Volhynian princes. 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. Upon the family's extinction in the 17th century, Ostroh passed to the princes Lubomirski. In the 16th century, first East Slavic books, notably the Ostrog Bible, were printed there. The town formerly had a sizable Jewish community, whose rabbis included Kalonymus Haberkasten, Samuel Edels, and Solomon Luria. A Jewish community existed in Ostroh already in the first half of the 15th century and perhaps earlier. At the beginning of WWII two thirds of the town's population of ca. 15,000 was Jewish. In Breslov Chassidus, this town has some significance, as it was the place where Rebbe Nachman's magnum opus, "Likutey Moharan" (Anthology of Our Teacher, Rebbe Nachman) was first printed.