Abandoned, lost or otherwise discarded fishing gear represents a substantial proportion of global marine plastic pollution and can cause significant environmental and socio-economic impacts. Yet little is known about its presence in, and implications for, freshwater ecosystems, or its downstream contribution to plastic pollution in the ocean. This study documents fishing gear related debris in one of the world's largest plastic pollution contributing river catchments, the Ganges. Riverbank surveys conducted along the length of the river, from the coast in Bangladesh to the Himalaya in India, show that derelict fishing gear density increases with proximity to the sea. Fishing nets were the main gear type by volume and all samples examined for polymer type were plastic. Illegal gear types and restricted net mesh sizes were also recorded. Socio-economic surveys of fisher communities explored the behavioural drivers of plastic waste input from one of the world's largest inland fisheries and revealed short gear lifespans and high turnover rates, lack of appropriate end-of-life gear disposal methods and ineffective fisheries regulations. A biodiversity threat assessment identified the air-breathing aquatic vertebrate species most at risk of entanglement in, and impacts from, derelict fishing gear; namely species of threatened freshwater turtle and otter, and the endangered Ganges river dolphin. This research demonstrates a need for targeted and practical interventions to limit the input of fisheries related plastic pollution to this major river system and ultimately, the global ocean. The approach used in this study could be replicated to examine the inputs, socio-economic drivers and ecological impacts of this previously uncharacterised but important source of plastic pollution in other major rivers, worldwide.