In 1800, the population of Japan was just over 30 million, a figure which would grow by just two million in the first half of the 19th century. However, with the fall of the Tokugawa shogunate and the restoration of the emperor in the Meiji Restoration of 1868, Japan would begin transforming from an isolated feudal island, to a modernized empire built on Western models. The Meiji period would see a rapid rise in the population of Japan, as industrialization and advancements in healthcare lead to a significant reduction in child mortality rates, while the creation overseas colonies would lead to a strong economic boom. However, this growth would slow beginning in 1937, as Japan entered a prolonged war with the Republic of China, which later grew into a major theater of the Second World War. The war was eventually brought to Japan's home front, with the escalation of Allied air raids on Japanese urban centers from 1944 onwards (Tokyo was the most-bombed city of the Second World War). By the war's end in 1945 and the subsequent occupation of the island by the Allied military, Japan had suffered over two and a half million military fatalities, and over one million civilian deaths.

The population figures of Japan were quick to recover, as the post-war “economic miracle” would see an unprecedented expansion of the Japanese economy, and would lead to the country becoming one of the first fully industrialized nations in East Asia. As living standards rose, the population of Japan would increase from 77 million in 1945, to over 127 million by the end of the century. However, growth would begin to slow in the late 1980s, as birth rates and migration rates fell, and Japan eventually grew to have one of the oldest populations in the world. The population would peak in 2008 at just over 128 million, but has consistently fallen each year since then, as the fertility rate of the country remains below replacement level (despite government initiatives to counter this) and the country's immigrant population remains relatively stable. The population of Japan is expected to continue its decline in the coming years, and in 2020, it is estimated that approximately 126 million people inhabit the island country.

The median age of the population in Japan has steadily been increasing since 1950 and is projected to be around 47.7 years old in 2020. As of 2021, the median age of Japan is the second highest in the world, behind the Principality of Monaco. The elderly in Japan An improved quality of life and regular health checks are just two reasons why Japan has one of the highest life expectancies in the world. The life expectancy from birth in Japan improved significantly after World War II, rising 20 years in the decade between 1945 and 1955. As life expectancy continues to increase, Japan expects difficulties caring for the older generation in the future. Shortages in the service sector are already a major concern, with demand for nurses and care workers increasing. Fertility and birth rates The fertility rate among Japan’s population has been around 1.4 children per woman since 2010. Apart from a small baby boom in the early seventies, the crude birth rate of Japan has been declining since 1950 and is expected to be as low as 7.5 births per thousand people in 2020. With falling birth rates and such a large share of its inhabitants reaching their later years, Japan’s total population is expected to continue declining.

The fertility rate of a country is the average number of children that women from that country will have throughout their reproductive years. From 1800 until 1865, Japan's fertility rate grew quite gradually, from 4.1 children per woman, to 4.8. From this point the fertility rate drops to 3.6 over the next ten years, as Japan became more industrialized. Towards the end of the nineteenth century, Japan's fertility rate grew again, and reached it's highest recorded point in the early 1920s, where it was 5.4 children per woman. Since this point it has been gradually decreasing until now, although it did experience slight increases after the Second World War, and in the early 1970s. In recent decades Japan's population has aged extensively, and today, Japan has the second oldest population and second highest life expectancy in the world (after Monaco). In contrast to this, Japan has a very low birth rate, and it's fertility rate is expected to fall below 1.4 children per woman in 2020.