Megacities contain at least 10 million people whose wellbeing largely depends on ecosystem services provided by remote natural areas. What is, however, most often disregarded is that nature conservation in the city can also contribute to human wellbeing benefits. The most common mind set separates cities from the rest of nature, as if they were not special kinds of natural habitats. Instead, awareness that urban systems are also nature and do host biodiversity and ecosystem services opportunities, should push urban people towards increased urban forest conservation and implementation strategies. This research estimated existing and potential, tree cover, and its contribution to ecosystem services in 10 megacity metropolitan areas, across 5 different continents and biomes. We developed estimates for each megacity using local data to transform i-Tree Eco estimates of tree cover benefits to reductions in air pollution, stormwater, building energy, and carbon emissions for London, UK. The transformation used biophysical scaling equations based on local megacity tree cover, human population, air pollution, climate, energy use, and purchasing power parity. The megacity metropolitan areas ranged from 1173 to 18,720 sq km (median value 2530 sq km), with median tree cover 21%, and potential tree cover another 19% of the city. Megacities had a median tree cover density of 39 m²/capita, much smaller than the global average value of 7800 m²/capita, with density lower in desert and tropical biomes, and higher in temperate biomes. The present median benefit value from urban trees in all 10 megacities can be estimated as $482 million/yr due to reductions in CO, NO2, SO2, PM10, and PM2.5, $11 million/yr due to avoided stormwater processing by wastewater facilities, $0.5 million/yr due to building energy heating and cooling savings, and $8 million/yr due to CO2 sequestration. Planting more trees in potential tree cover areas could nearly double the benefits provided by the urban forest. In 2016 there were 40 megacities, totaling 722 million residents, nearly 10% of the human population, who would benefit from nature conservation plans where they work and live. Nature conservation strategies in megacities should work to sustain and grow the benefits of the urban forest, and improve accounting methods to include additional ecosystem services provided by the urban forest.