This paper examines multilateral cooperation in Northeast Asia (NEA) through the Greater Tumen Initiative (GTI) as a case of micro-regional cooperation. Drawing on functionalism and micro-regionalism, it explores how cooperation spillover from nonpolitical to political spheres can be facilitated by considering participant diversity, agenda comprehensiveness, and the complexity of cooperation. There is an overall positive trajectory of change as the GTI actively fosters cooperation spillover by expanding its committees and partnerships. However, in terms of committee activities, although its agendas demonstrate increasing comprehensiveness, participant diversity and collaboration complexity remain low in general. The partnerships are in their nascent stages of cooperation, exhibiting limited spillover, except for the NEA Local Cooperation Committee. Moreover, the challenge in evaluating accomplishments within the framework of the GTI arises from the apparent scarcity of discernible achievements. To revitalize multilateral cooperation, the GTI should enhance actor diversity, emphasizing private sector participation; broaden discussion agendas rather than exclusively focus on specific projects until institutional stability is established in cooperation; and enhance governance mechanisms and foster cooperation among internal organizations, currently vulnerable to disruptions, to ensure progressive and sustained collaborative efforts. Through these enhancements, it is anticipated that tangible outcomes will be uncovered and, ultimately, foster micro-regional multilateral cooperation that can significantly contribute to peace and prosperity in the Northeast Asia region.