Achasan (Korean: 아차산) is a hill that sits between the counties of Gwangjin District and Guri in South Korea. It has an elevation of 287 m (942 ft).

There is an extensive system of trails on the Achasan and Yongsan mountains with numerous entrances. People often hike from Yongmasan (용마산) and go to Achasan, or vice versa. Either trail takes about two hours. It has an elevated view of the Han River and of Seoul. At the halfway observation deck and the summit, there is a view of the upper stream of the Hangang River and the cities of Guri and Hanam. From Achasan Fort the Han River is visible almost in its entirety as it flows through Seoul). This is the closest vantage point for viewing the Hangang River from above.

• List of mountains in Korea

• Travel Highlights by Visit Korea Archived 2012-01-23 at the Wayback Machine, 2012