The Battle of Kostiuchnówka was a World War I battle that took place July 4–6, 1916, near the village of Kostiuchnówka (Kostyukhnivka) and the Styr River in the Volhynia region of modern Ukraine, then part of the Russian Empire. It was a major clash between the Russian Army and the Polish Legions (part of the Austro-Hungarian Army) during the opening phase of the Brusilov Offensive.