Japan and South Korea face each other across the Korea Strait, and on almost every physical measure Japan is the larger of the two neighbours, but on income per person they are now level. Japan covers 377,975 km², about 3.8 times South Korea’s 100,472 km², and has almost 123 million people (2026) against South Korea’s roughly 52 million. On IMF estimates for 2026, South Korea’s nominal GDP per capita is about US$37,400, slightly ahead of Japan’s US$35,700, a lead that would have been unthinkable a generation ago. Japan’s capital is Tokyo, at the centre of the world’s largest metropolitan area; South Korea’s is Seoul, whose metropolitan region holds half the country. Japanese is Japan’s de facto national language, and Korean, written in the Hangul alphabet, is South Korea’s official language. The comparison between them is inseparable from their shared past: 35 years of Japanese colonial rule over Korea, which still shapes how the two governments talk to each other.
Fifty Kilometres of Water: Busan, Tsushima and the Korea Strait
The two countries have no land border. They meet at sea, in the Korea Strait, a channel about 200 km wide and on average only 90 to 100 m deep that links the Sea of Japan (which South Korea calls the East Sea) with the East China Sea. The Japanese island of Tsushima sits in the middle and splits the strait into a deeper western channel, up to 227 m, and the eastern Tsushima Strait. From Busan, South Korea’s second city and busiest port, to the nearest point of Tsushima is about 50 km, and from Tsushima it is about the same distance again to Iki Island and on towards Kyushu. Both countries limit their territorial waters here to 3 nautical miles rather than the usual 12, so that ships of any flag can pass freely.
That short gap has always been a route rather than a wall. In the ancient period the southern tip of the Korean Peninsula was the closest port for trade and cultural exchange between the Japanese islands and mainland Asia; Buddhism reached Japan across the strait from the Korean kingdom of Baekje in the 6th century. Today commercial ferries run from Busan and Geoje to Fukuoka, Tsushima, Shimonoseki and Hiroshima.
Further north, the sea holds the one territory the two governments openly dispute. The Liancourt Rocks, called Dokdo in Korean and Takeshima in Japanese, are two main islets and surrounding rocks with a total area of about 0.19 km² and a highest point of 168.5 m. South Korea administers them as part of Ulleung County in North Gyeongsang Province and keeps a police detachment, lighthouse and helicopter pad there; Japan claims them as part of Okinoshima in Shimane Prefecture, into which it incorporated them in 1905. The nearest Korean island, Ulleungdo, is 87.4 km away and Japan’s Oki Islands are 157 km away. The name of the surrounding water is also contested, as our guide to the Sea of Japan explains.
From the Imjin War to the 1965 Treaty
The history that divides the two countries runs through three dates. Between 1592 and 1598 Toyotomi Hideyoshi launched two invasions of Korea, known in Korea as the Imjin War; Japanese armies occupied much of the peninsula before Ming Chinese reinforcements and the Joseon navy forced a withdrawal. Three centuries later Japan forced Emperor Gojong to abdicate in 1907, and annexed the country outright with the Japan–Korea Treaty signed on 22 August 1910. Colonial rule lasted until Japan’s surrender on 15 August 1945, a period in which the Korean language and history were suppressed and, by one estimate, as many as 7.8 million Koreans were conscripted into forced labour, military service or sexual slavery as so-called comfort women.
Liberation did not bring unity. The peninsula was divided, the Republic of Korea was proclaimed in the south in August 1948, and the Korean War of 1950 to 1953 ended only in an armistice, leaving the heavily fortified line described in our article on the Korean Demilitarized Zone. Japan and South Korea did not establish diplomatic relations until the Treaty on Basic Relations was signed on 22 June 1965. Under the accompanying agreements Tokyo provided US$300 million and extended a further US$200 million in credit. Japan holds that all wartime claims were settled by that package; South Korean courts have ruled that individual victims kept the right to sue, and the Supreme Court in Seoul has ordered Nippon Steel and Mitsubishi Heavy Industries to compensate former forced labourers. A 2015 agreement on comfort women, reached when Fumio Kishida was Japan’s foreign minister, was effectively set aside in November 2018, when the Moon Jae-in government shut down the Japanese-funded foundation created to carry it out.
Zainichi Koreans, the 2002 World Cup and the 2019 Trade Row
People have moved across the strait in large numbers. Japan’s justice ministry recorded 407,341 South Korean nationals as residents of Japan in 2025, many of them Zainichi Koreans whose families arrived during the colonial period and stayed after 1945. Tourism has swung with politics: Japanese visitors to South Korea halved from 3.5 million in 2012 to 1.8 million in 2015, while South Korean visitors to Japan doubled from 2 million to 4 million over the same years.
Culture has crossed too, in both directions. After 1945 South Korea banned Japanese films, music, television and manga; the ban began to be lifted under President Kim Dae-jung in 1998, the same year Prime Minister Keizō Obuchi offered a “heartfelt apology” for colonial rule, and Japanese CDs and DVDs were allowed in from 2004. In the other direction the Korean Wave has carried groups such as TVXQ, Big Bang, Girls’ Generation, Twice and BTS into the Japanese market. In 2002 the two countries co-hosted the FIFA World Cup, the first held in Asia and the first with more than one host, with the final played in Yokohama.
Trade is large but has been used as leverage. Annual trade between the two was US$77.2 billion in 2024. In July 2019 Japan tightened export licensing on materials used by South Korean chipmakers, citing concerns over South Korea’s controls on strategic goods, though the move was widely seen in Seoul as a response to the forced-labour rulings; Seoul took the case to the World Trade Organization and gave notice that it would leave the GSOMIA intelligence-sharing pact signed in 2016. South Korean travel to Japan fell by about 65% in the last three months of 2019. The dispute ended in March 2023, when South Korea withdrew its complaint and Japan lifted the restrictions, and in August 2023 President Yoon Suk Yeol and Prime Minister Kishida joined US President Joe Biden in the Camp David Principles.
Mount Fuji at 3,776 m Against Hallasan at 1,950 m
On the ground, the two countries share a mountainous character but differ in scale. Japan is an archipelago of 14,125 islands, led by Hokkaido, Honshu, Shikoku, Kyushu and Okinawa. Its terrain is mostly rugged and mountainous, 67% of it forest and only about 14% farmland, which pushes most people and farms onto coastal plains. It lies on the Pacific Ring of Fire, with 111 active volcanoes and frequent earthquakes, including the 1923 Great Kanto earthquake, which killed more than 140,000 people, and the 2011 Tōhoku earthquake, the largest in its recorded history. Its highest peak is Mount Fuji, an active stratovolcano on Honshu about 100 km south-west of Tokyo, whose summit reaches 3,776 m; it last erupted in 1707–08. Lake Biwa is its largest freshwater lake.
South Korea occupies the southern part of a peninsula, bordered by North Korea along the Demilitarized Zone and by the Yellow Sea to the west. Its terrain is also mostly mountainous and largely not arable; lowlands make up only about 30% of the land. The land falls into four broad regions: high ranges and narrow plains in the east, broad coastal plains and river basins in the west, mountains and valleys in the south-west, and the wide basin of the Nakdong River in the south-east. About 3,000 islands lie off the western and southern coasts. The largest, Jeju, about 100 km off the south coast, carries the country’s highest point, Hallasan, a dormant volcano of 1,950 m, just over half the height of Fuji.
Tokyo’s 37 Million, Seoul’s Half a Nation and a Dead Heat on Income
Both countries are highly urban and ageing fast: Japan has the oldest population, South Korea the lowest birth rate. Greater Tokyo, with 37.4 million people in 2024, is the biggest metropolitan area in the world; about half of all South Koreans live in the Seoul metropolitan area, with Busan, Daegu and Incheon the next largest cities. Japan’s total fertility rate is 1.2 and its median age of 48.4 is the highest of any country, and its population is already falling. South Korea’s fertility rate fell to 0.72 in 2023 and, despite a rise to 0.80 in 2025, is still the lowest in the world.
Politically, Japan is a constitutional monarchy under its 1947 constitution, with Emperor Naruhito on the throne since 2019 and power held by the National Diet. South Korea is a presidential republic under the 1987 constitution that followed the June Democratic Struggle and ended authoritarian rule; President Yoon’s declaration of martial law in December 2024, the first since 1980, tested those institutions.
The economies grew along similar lines, from post-war ruin to export-led manufacturing. Japan became the world’s second-largest economy before its asset bubble burst in the early 1990s; it is now the fourth-largest by nominal GDP, and Toyota is the world’s largest carmaker by production. South Korea’s rise, which it calls the Miracle on the Han River, came later and faster, led by Samsung, LG and Hyundai; semiconductors and related AI hardware now make up about 40% of its exports. Decades of Japanese stagnation and Korean growth have brought income per person to near parity, with the IMF now putting South Korea marginally ahead. To see how the two capitals compare up close, read our profile of Tokyo, Japan’s capital, and set it beside Seoul across the strait.