The world's biggest mega-city in 2025 that's 4 times bigger than London

Mega-cities are popping up all around the world as Earth's population increases. It currently stands at just over 8 billion, and it's expected to keep growing. And while India is now the most populous country with nearly 1.5 billion people, it’s not home to the world’s largest city. That title belongs to Japan, where around 123 million people live, and nearly 40 million of them are packed into one massive city.

That mega-city is of course Tokyo, which is the biggest city on Earth in 2025, with a population of 37,036,200, according to the World Population Review. Although slightly down from 37,115,000 in 2024, it still dwarfs other cities around the world. In contrast, London ranks 37th with a population of 9,840,740. This means Tokyo is nearly four times bigger in terms of people.

The city also covers an area of 13,452 square kilometres, according to BBC Science Focus. That space is filled with an average of 2,642 people per square kilometre, creating big challenges for housing and transport.

Because of space shortages, some locals have moved into tiny apartments as small as nine square metres, especially younger residents wanting to live closer to the city centre.

Getting around can be tough too. Roads are often clogged, and public transport is usually full, although it’s widely praised for its punctuality and coverage.

Despite these challenges, Tokyo still ranks highly as a place to live. According to Time Out’s Best Cities Survey 2025, Tokyo came 31st overall.

Residents rated it highly for public transport, great restaurants and overall happiness, with 70% saying the city makes them feel good.

The city’s popularity with tourists is also growing thanks to the cheap yen, new developments like Tokyu Plaza Harajuku Harakado, and massive complexes such as Azabudai Hills.

These combine green spaces with shopping, art, and unique architecture.

According to CN Traveler, top things to do in Tokyo include visiting Shibuya Crossing, often called the world’s busiest pedestrian intersection.

The Tokyo Skytree is another popular spot - the tallest free-standing tower in the world at 2,080 feet, offering sweeping views of the city.

For those after some quiet, Shinjuku Gyoen is a peaceful 144-acre garden that features Japanese, French, and English-style landscaping, perfect for cherry blossom season.

History lovers can also visit Senso-ji Temple, Tokyo’s oldest temple. Japan’s capital is also famous for its food, especially sushi (some call Tokyo the birthplace of sushi), so there is plenty to do for everyone.