Topicality. The study of the development mechanisms of regional economic systems is an urgent task in connection with the reform of decentralization in Ukraine, which creates new opportunities for economic development of territories. The relevance of this study is determined by: the State Strategy for Regional Development 2021-2027 (Strategic Objectives � 1-3) [1], the Concept of Local Government Reform [2], the National Economic Strategy for the period up to 2030 [3]. In particular, these documents emphasize [1] the need to build competitive regions and functional areas by involving all actors in development and overcoming the inequality of socio-economic development of regions. Aim and tasks. The aim of the article is to determine the conceptual foundations and methodological principles of building and increasing the capacity of economic systems of territorial communities in Ukraine, as well as the formation of mechanisms for implementing relevant principles of forming and ensuring effective functioning of regional economic systems. Research results. The basic principles according to which the regional economy should develop are revealed. The problem of regional economic systems (RES) management from the point of view of network management is considered. The hypothesis of a new economic system is put forward. The connection of the institutional basis of RES with the goals of sustainable development is shown. The institutional basis for ensuring the effective development of regional economic systems in the context of local government reform is substantiated and the current principles are formulated, according to which territorial economic systems should function in accordance with the requirements of national and European regulations. Hypotheses about the need to supplement the existing list of principles of functioning of regional economic systems with the principles of structural and institutional support of their development are proved. The mechanisms of realization of the principles of functioning of regional economic systems in modern conditions are substantiated, which include instruments: tax, credit, financial-budgetary, organizational-infrastructural, property-legal. Measures are proposed to ensure effective economic cooperation of communities. Conclusion. The successful functioning of communities depends primarily on the activities of regional economic systems. The joint implementation of economic projects by communities is most conducive to their cooperation in all areas. The activities of the regional economic system must comply with the principle of structure and the principle of economic cohesion (economic solidarity). In the long run, these principles should lead to the emergence of a new solidarity-based economic paradigm.