The Sustainable Development Goals (SDGs) emphasise the importance of health as a foundation for sustainable futures, with SDG 3 targeting the promotion of good health and wellbeing. However, existing frameworks often overlook the crucial role of the environment in shaping mental health. This paper introduces the Environmental Determinants of Mental Health (EDMH) framework, which integrates natural, built, and social environments into the biopsychosocial model. The EDMH framework conceptualises mental health as an emergent outcome of reciprocal interactions between individuals and their environments. Direct pathways such as air pollution, noise, and heat exposure, alongside indirect pathways including social isolation, housing insecurity, and perceived safety, are identified as key influences on mental health outcomes. Moderators such as socioeconomic status and culture, and mediators including perceptions and coping strategies, further shape these relationships. By synthesising evidence across environmental psychology, urban planning, and public health, this theoretical model fills a critical gap in existing approaches. It provides a structured foundation for empirical testing, policy alignment, and the development of context-specific interventions. Importantly, the EDMH model extends SDG 3 by broadening the focus beyond healthcare delivery to include the environmental and ecological conditions that underpin mental health resilience. By positioning the environment as central to mental wellbeing, the framework establishes a pathway for interdisciplinary action, offering a robust lens for future research, practice, and policy to support global health equity.