On the basis of previously unknown documents, the author researches the course of the military operation of the Northern Group of the UPR Armed Forces against the Red Army units in Korosten region in March 1919. The relevance of the study is determined by the complete lack of coverage of the Korosten operation in historical literature, although the successful acquisition of Korosten by Ukrainian troops was the first significant military success of the UPR Army during its second war with the Soviet Russia (1918–1920). The publication is based on the documents of the General Staff of the UPR Army, which are currently stored in the funds of the Central State Archives of Supreme Bodies of Power and Government of Ukraine (Kyiv, Ukraine), as well as materials of the Headquarters of the Ukrainian Soviet Front stored in the Russian State Military Archives (Moscow, Russia). The vast majority of these archival documents are introduced to scientific circulation for the first time. The author highlights the goals and tasks which the Ukrainian Command sought to achieve during the operation; determines the ratio of the forces of the belligerents; analyzes the composition and number of forces involved on both sides; reveals the main stages of the military operation on the acquisition of Korosten in March 1919, the role of the units and squads of the UPR Army during the course of the seizure of the town. The author reconstructs in details the course of the fighting crowned with the victory of the Ukrainian weapon; determines the place of these events in the strategic line of the Ukrainian Command; shows their influence on the further course of the military operations. It is emphasized that the successful implementation of the Korosten operation was of extreme strategic importance, providing the Ukrainian Army with the opportunity to avoid the invasion from the north and deploy an active offensive operation in the eastern direction at the same time. The victory of Korosten in March 1919 not only allowed the Ukrainian troops to intercept the strategic initiative but also became the first step towards organizing a general counter-offensive by the UPR Army against Soviet troops in the Right-Bank Ukraine in the spring of 1919.