Diseases caused by pathogens that affect animals, humans, and their shared environment (zoonoses) impact regional and global health security. Context-specific recommendations could help control animal-to-human pathogen spillover and mitigate the risk of epidemics and pandemics. Our objective is to share the codevelopment of a guidebook to counter zoonotic spillover in Southeast Asia and how participatory approaches and implementation science guided One Health actor groups towards a common goal. The project was a collaboration between the International Network for Governmental Science Advice Asia and the US National Academies of Sciences, Engineering, and Medicine.