Seoul is the capital and largest city in  South Korea. It's considered a megacity because it has a population of over ten million people, with nearly half of its 10,208,302 people residing in the National Capital Area (which also includes Incheon and Gyeonggi). Seoul, South Korea. The Seoul National Capital Area is the second-largest in the world at 233.7 square miles and an average elevation of just above sea level at 282 feet.