Abstract: The subject of this article is the entity of the relations that existed between the main ethnic groups (primarily Belarusians and Poles) on the territory of the Bialystok region of the BSSR, most of which was transferred to postwar Poland by the Soviet Union. The chronological frame of the study is the period from the time of the end of the German occupation up to the peak of the Polish nationalist underground activities in the region (1944-1950). The choice of the postwar period makes it possible to analyze the interethnic relations in all their acuteness, as characteristic of a period of unstable borders, crises caused by ethnic and ideological conflicts, and weakness of the authorities under the conditions of restoring a peaceful life after the cruelest wartime turmoil. Within the framework of this research based on archival evidence, the author analyzes such key phenomena for the understanding of the Polish-Belarusian relations as the policy of the Polish armed anti-Soviet underground against the Belarusian population and the reaction of Belarusians and Poles to the postwar settlement of the region in the context of the establishment of the socialist system. The conclusions presented by the author indicate that the political and interethnic conflicts in the post-war years were closely intertwined on the territory of Western Belarus. The article demonstrates their influence on the principles of coexistence of the two main national communities in the region. Their tracks are still making themselves felt today, materializing in the form of ethno-confessional ideological notions, which to this day echo in Polish regions tightly inhabited by Belarusians.