Seosan is a city located in the South Chungcheong Province of South Korea. Situated at the northwestern end of the province, it has a population of approximately 175,000 as of the 2017 census. The city is geographically bounded by Dangjin, Naepo New Town, Yesan-gun, and Hongseong-gun to the east, while Taean-gun and the Yellow Sea lie to its west.
The city serves as a major transportation hub on the west coast of the country. It provides access to several key routes, including the Seohaean Expressway and the Daejeon-Dangjin Expressway. Additionally, National Highways No. 29, 32, 38, and 45 intersect within the city limits. Seosan is located approximately 125 km south of Seoul, 159 km northwest of Daejeon, and 34 km northwest of Naepo New Town.