Achasan (Korean: 아차산) is a hill that sits between the counties of Gwangjin District and Guri in South Korea.

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,[1] or vice versa.

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).