ABSTRACT Sewage pollution is a major challenge for the Southeast Asian region, particularly in Indonesia, Malaysia, and Thailand. The increasing volume of domestic and industrial waste, as well as the lack of public understanding in maintaining environmental quality, has worsened environmental conditions in the region. To address these issues, the three countries formed cooperation through the Indonesia-Malaysia-Thailand Growth Triangle (IMT-GT) and implemented the Green City Action Plan (GCAP) as a sustainable development strategy. This research shows the effectiveness of IMT-GT cooperation in dealing with waste pollution through the implementation of GCAP, using the concept approach of Sub-Regionalism and Green Economy. This research uses a qualitative method with secondary data collection techniques from various academic literature, official documents, and policy reports. The results show that IMT-GT cooperation has had a positive impact on waste management, but still faces challenges in regulatory harmonization, infrastructure restrictions, and lack of community participation. Therefore, increasing synergy between countries, strengthening environmental policies, and investing in green technology are important steps to achieve better and sustainable waste management in the IMT-GT region.