The development of the country's regions is ensured by realizing their resource potential. Interregional integration is especially important, in particular within the framework of the developing model of inter-country cooperation – the Union State of Belarus and Russia. A literary review of research works on problematic issues of interregional cooperation development, improvement of methodological approaches to assessing its effectiveness, and development of interregional integration mechanisms is presented. The research uses the methods of structural, and system analysis and index method. The paper presents the interaction results of Brest region of the Republic of Belarus with the regions of the Russian Federation, presents the dynamics analysis of the results achieved and forms a number of proposals for further development of interregional cooperation as a mechanism for competitiveness increase of the allied countries. The growth of foreign trade volume between Belarus and Russia in 2005-2021 was noted. The average annual growth rate for this indicator was 18.2% in Belarus and 13.6% in Russia. By the end of 2022, a record figure of 50 billion USD was registered for mutual trade turnover of Belarus and Russia. Cooperation in trade and economic, scientific and technical, ecological, socio-humanitarian and cultural areas of Brest region and the regions of the Russian Federation, as well as formation of joint production and technological complexes and development of cooperation forms in scientific and educational sphere in the concept of creating a single scientific and technological, and educational space are identified as promising for further development of interregional cooperation for 2023.