This article deals with the Economic Network of the Three Seas Regions as a legal instrument of macro-regional development. The author, referring to the acquis of administrative law and EU law, highlights new opportunities arising from legal solutions in the field of regional development law. To attain the research objective, the author has analysed the legal prospects of macroregional development in the Three Seas Initiative area. Based on that analysis, a conclusion has been formulated that the use of network administration processes, which will ultimately lead to establishing the Economic Network of the Three Seas Regions, can serve as a legal instrument fostering regional development in the reference area. While the reference Network will likely intensify international local-government cooperation, it will notably function as an instrument of the European network administration. The development of network administration connected with the Three Seas Initiative will contribute to shaping the sustainable regional development law for that area, covering most of its regions.