Cambodia is a rapidly developing country in Southeast Asia, a region forecast as an outsized source of plastic pollution into the world’s oceans. However, to date there has been no large-scale assessment of plastic pollution in the environment within the country. This study is a comprehensive assessment of plastic items and hotspots across 243 coastal, river and inland sites along the entire coastline of Cambodia, recording 46,927 items in transects throughout the study area. Plastic densities along coastlines were more than ten times those on riverbanks and more than twenty times that of inland areas. Averaging 70 plastic items per metre of coastline, Cambodia has among the highest average loads of whole plastic items recorded on coastlines globally. Single-use plastic bottles were the most abundant whole item recorded, overrepresented by drinking water bottles. Polystyrene foam was the most common fragmented item. We provide specific recommendations to reduce plastic in Cambodia’s environment.