Climate change and deteriorating air quality are concurrently escalating in Southeast Asia; however, the precise nature of their regional interrelation remains inadequately understood. This study uses a mixed-methods approach, combining a systematic review of existing research, an analysis of quantitative secondary data, and a qualitative examination of policy. Data on PM2.5 concentrations from WHO and IQAir show that all nine ASEAN countries monitored consistently exceed the WHO annual guideline of 5 µg/m³. These results show that climate change and air pollution in Southeast Asia are both worsening problems that need immediate, coordinated policy responses that benefit all four sectors: energy, transportation, land use, and agriculture.