According to the United Nations' 'World Urbanization Prospects 2018' report, Tokyo, Japan, was indeed the most populous city in the world. Its vast metropolitan area, encompassing the city and surrounding prefectures, had an estimated population exceeding 37 million people. This immense population is a result of decades of urbanization, economic growth, and a highly developed infrastructure that supports its dense urban sprawl, making it a global megacity.
While Delhi, India, is a rapidly growing megacity and one of the world's largest, its population in 2018 was significantly less than Tokyo's. Shanghai, China, is another massive global city and a major economic hub, but it also did not surpass Tokyo in terms of overall metropolitan population in 2018. Therefore, Tokyo held the top spot for urban population.