Urbanisation in Southeast Asia is unfolding at an unprecedented pace, driving the expansion of infrastructure-intensive grey cities at the cost of natural ecosystems. This rapid loss of urban nature raises urgent concerns for environmental sustainability, climate resilience, and public health. Here, we examine the relationships between urban nature, economic productivity, and population health across rapidly developing cities in Southeast Asia. While the concept of nature in cities from Global North perspectives emphasises integrating green spaces, ecosystem services, and biodiversity into well-planned, post-industrial and often high-income settings, such models are not easily transferable to many Southeast Asian cities, which face distinct constraints. This include the need to provide basic infrastructure and services to rapidly growing populations, often with weaker governance and lower financial resources. As such, their urbanisation trajectories face direct challenges and trade-offs between economic, environmental and human health dimensions. We explore: what extent does investing in urban nature in Southeast Asia represent a trade-off, or synergy, with economic growth and public health outcomes? Drawing on multiple examples from cities in the region, we explore associations between urban nature extent and ecological integrity, and economic productivity and population health outcomes. We highlight how relationships between people and nature extend beyond cities’ administrative boundaries, and that well-integrated urban nature is a key determinant of human survival, health and well-being. We argue that underinvestment in urban ecosystems can erode long-term urban sustainability and public health priorities. We call for a reframing of urban development strategies in Southeast Asia—one that positions urban nature as a critical component for building future-ready resilient, inclusive, and ecologically sustainable cities in Southeast Asia.