Seoul proper is noted for its population density, which is almost twice that of New York and eight times greater than Rome. Its metropolitan area was the most densely populated in the OECD in Asia in 2012, and second worldwide after that of Paris. As of 2015, the population was 9.86 million, in 2012, it was 10,442,426.
 As of the end of June 2011, 10.29 million Republic of Korea citizens lived in the city. This was a .24% decrease from the end of 2010. The population of Seoul has been dropping since the early 1990s, the reasons being the high costs of living, urban sprawling to Gyeonggi region's satellite bed cities and an aging population.
As of 2016, the number of foreigners living in Seoul was 404,037, 22.9% of the total foreign population in South Korea. As of June 2011, 186,631 foreigners were Chinese citizens of Korean ancestry. This was an 8.84% increase from the end of 2010 and a 12.85% increase from June 2010. The next largest group was Chinese citizens who are not of Korean ethnicity; 29,901 of them resided in Seoul. The next highest group consisted of the 9,999 United States citizens who were not of Korean ancestry. The next highest group were Taiwanese citizens, at 8,717.
The two major religions in Seoul are Christianity and Buddhism. Other religions include Muism (indigenous religion) and Confucianism. Seoul is home to one of the world's largest Christian congregations, Yoido Full Gospel Church, which has around 830,000 members.
Seoul is home to the world's largest modern university founded by a Buddhist Order, Dongguk University.