By 2026, the Tokyo urban area is expected to lose its long‑held title as the world’s largest city by population, according to United Nations projections and national statistics offices, with demographic data pointing to Delhi as the most likely contender to become the new number one megacity.


The shift, driven by slower growth and ageing in Japan and rapid urbanisation in South Asia, marks a symbolic turning point in the geography of global cities. Demographers, urban planners and economists say the handover underscores deeper changes in where economic growth, climate risk and infrastructure pressures are most concentrated.


Delhi’s expanding urban skyline. UN data indicate that the Delhi urban agglomeration is on track to overtake Tokyo as the world’s largest city by 2026. Image: Wikimedia Commons, CC BY-SA 4.0.

How Do We Decide Which City Is the “Biggest”?

Determining the biggest city in the world is less straightforward than it appears. International agencies such as the United Nations World Urbanization Prospects distinguish between city proper, urban agglomeration and metropolitan area. The rankings that placed Tokyo at the top for decades generally refer to the broader urban agglomeration, which includes continuous built‑up areas extending beyond municipal boundaries.


The UN’s 2022 and 2024 updates projected Tokyo’s urban agglomeration to stabilise or decline slightly from around 37 million residents, while Delhi’s was expected to exceed that figure before the end of the 2020s. Preliminary figures from India’s statistical authorities and independent demographic researchers, cited by multiple academic journals, suggest that the crossover point could arrive as early as 2026 if current trends continue.


“Tokyo remains an enormous economic hub, but in demographic terms it has already passed its peak,” said a population specialist at a UN regional office, speaking on background because updated 2025 tables had not yet been formally released. “South Asian megacities, particularly Delhi, are where the growth is now most visible.”


Tokyo’s Long Reign at the Top

Tokyo has been widely cited as the world’s largest city since at least the late 1960s, supported by Japan’s post‑war economic boom and large‑scale internal migration. By the early 2000s, UN datasets estimated the Tokyo urban agglomeration at more than 35 million residents, putting it well ahead of megacities such as Mexico City, São Paulo and New York.


Japan’s rapid ageing and low birth rate, however, have reshaped its demographic profile. According to the Statistics Bureau of Japan, the country’s total population has been declining since 2010, and the Tokyo metropolitan region has moved from fast growth to near‑stagnation. While international migration has buffered the decline, most forecasts agree that the Tokyo agglomeration will plateau or shrink modestly over the coming decades.


“Population size is no longer Tokyo’s primary advantage,” said a Tokyo‑based urban economist in a recent study from a Japanese policy institute. “Productivity, innovation and livability are now more decisive than sheer numbers.”

Delhi and the Race Among Emerging Megacities

Among the emerging megacities, Delhi has been the most frequently named successor to Tokyo. The UN’s 2018 and 2022 World Urbanization Prospects editions projected Delhi’s population to rise sharply, reflecting India’s broader demographic momentum and rapid urbanisation in the National Capital Region.


India’s most recent national estimates, combined with satellite‑based analyses of built‑up areas published in journals such as Remote Sensing of Environment, suggest that Delhi’s effective urban footprint extends far into neighbouring states, blurring the distinction between the capital and surrounding satellite towns. When these are included, researchers say the Delhi agglomeration could surpass 37 million residents around 2026.


Other cities are also in contention, depending on the definitions used. The greater Jakarta region in Indonesia, sometimes referred to as Jabodetabek, and the Guangzhou–Foshan urban area in China are both cited in academic literature and by organisations such as Demographia as among the world’s largest continuous urban regions. Some scholars argue that Lagos, Nigeria, may also be undercounted in official statistics, with informal settlements making precise measurement difficult.


“Delhi is the likeliest candidate using UN definitions,” said a South Asia urban studies researcher at a European university. “But if we shift the lens to functional urban regions or commuting zones, you might see different cities, particularly in China and Southeast Asia, competing for the top spot.”


Why the “World’s Biggest City” Is Still Contested

Not all experts agree that any single city can be definitively labelled the world’s largest. The Organisation for Economic Co‑operation and Development (OECD), the World Bank and national statistics offices each use slightly different criteria for what constitutes a metropolitan area, complicating comparisons.


  • Some rankings focus on administrative city limits, which can understate the size of sprawling megacities.
  • Others use continuous built‑up area, aligning more closely with what residents experience as the city.
  • Functional urban areas, based on commuting patterns and economic linkages, can produce yet another set of rankings.

Data quality also varies. While Japan and many high‑income economies maintain frequent, detailed surveys, some fast‑growing cities in lower‑ and middle‑income countries rely on less frequent censuses and partial estimates. Remote sensing and big‑data approaches have begun to close some of these gaps, but they often come with their own methodological debates.


“Whether Delhi passes Tokyo in 2026 or a year or two later is less important than the structural reality that the centre of urban gravity is shifting,” said a demographer at an Asian think tank. “The precise date may remain open to interpretation because of how we count and define cities.”


What the New Biggest City Means for Infrastructure and Climate

The emergence of a new biggest city carries practical implications beyond symbolism. Urban planners point out that megacities with more than 30 million residents face acute challenges in transport, housing, water supply and climate resilience, particularly in regions already exposed to extreme heat, flooding or air pollution.


Delhi, for example, regularly records some of the world’s highest concentrations of fine particulate matter, according to the World Health Organization. The city also experiences severe heatwaves and seasonal water stress, issues that researchers say could intensify as the population grows and climate change accelerates.


A Delhi Metro line running through a densely built district. Transport, housing and basic services face mounting pressure as the city’s population grows. Image: Wikimedia Commons, CC BY-SA 3.0.

Advocates of dense urbanisation argue that large cities can also deliver environmental benefits if managed well, concentrating services and reducing per‑capita emissions through public transport and energy‑efficient buildings. Tokyo is frequently cited in policy studies for its extensive rail network and relatively low car ownership compared with its income level.


Others caution that without substantial investment, expanding megacities may see widening inequality, with informal settlements and inadequate infrastructure affecting tens of millions of residents. Reports from the World Bank and UN‑Habitat stress the importance of coordinated metropolitan governance, land‑use planning and social protection to avoid what they describe as “unequal urbanisation.”


Economic Power, Soft Influence and Global Perception

The change in rankings also feeds into broader narratives about shifting global power. India’s government has highlighted the country’s large and youthful population as a competitive advantage, while Japanese policymakers have focused on productivity gains, technology and ageing‑society innovation to sustain economic performance with fewer people.


Some economists note that population alone does not determine a city’s global influence. New York, London, Singapore and other financial centres exert outsize economic impact despite being far smaller than the largest Asian megacities. Yet population size can shape investment priorities, infrastructure finance and international attention.


“Being labelled the world’s biggest city can draw both pride and scrutiny,” said an urban policy analyst at a multilateral development bank. “It may attract investment, but it also raises expectations about how that city manages congestion, pollution and social inclusion.”



A Symbolic Shift in the Urban Century

Whether the exact handover occurs in 2026 or within a slightly broader window, most major datasets point to a near‑term moment when Tokyo will no longer be the world’s largest city by population. The likely emergence of Delhi, or another South or East Asian megacity, at the top of the ranking reflects longer‑running trends in global demographics, economic development and urbanisation.


Researchers and planners say the transition underscores both the opportunities and pressures facing the world’s fastest‑growing urban regions. As the focus of global urbanisation shifts, the question for policymakers is less which city is biggest and more how the largest cities can remain liveable, resilient and inclusive for tens of millions of residents.