Diverting waste away from and zero waste to landfills are key sustainability policy aims of local and national governments around the world, particularly in countries with large waste footprints from rapid consumption growth and urbanisation. Segregation at the source of waste generation can offer a low-cost solution to urban waste footprints, yet segregation rates are low in many places, especially in the cities of developing economies. This paper studies a staggered randomised intervention offering training and education to citizens about waste segregation. Citizens in the city of Patna in India were given training on waste segregation at source, recycling and its environmental benefits in a large experimental intervention undertaken in collaboration with the city administration. Segregation-at-source increased substantially among households that received the intervention, and additional boosts to segregation arose from spatial spillovers, as the programme delivered at least a double-digit benefit-cost ratio. Citizen training, when effectively designed and implemented, does deliver a low-cost solution for the cities of developing countries to both reduce their waste footprint and enhance local environmental sustainability.