Gwanaksan (Korean: 관악산) is a mountain in southern Seoul, South Korea. Portions lie in the Gwanak District and Geumcheon District of Seoul; other portions lie in the neighboring cities of Anyang, Gyeonggi and Gwacheon.

The name gwanak means "hat-shaped peak," and refers to its gat-like profile. Thus Gwanaksan literally means "mountain of the hat-shaped peak". Together with Songaksan in Kaesong, Gamaksan in Paju, Uaksan in Pocheon, and Hwaaksan in Gapyeong, Gwanaksan was long considered one of the five representative peaks of Gyeonggi Province (gyeonggi o-ak).

The main campus of Seoul National University is located just northwest of the mountain, and the Gwacheon national government complex lies to the east. Also nearby is the Anyang Resort Area, a popular local tourist attraction. On the mountain itself lie numerous Buddhist temples, including Wongaksa. Most of the mountain slope is protected land. The mountain is very popular with older climbers, since it can easily be reached by Seoul's public transportation. Annual hiking traffic is close to 5 million people. The Seoul side was designated a city park in 1968.

Seoul National University Entrance (11.3 km, 4 hours) Siheung-dong (11 km, 3 hours, 30 minutes) Anyang amusement park (12 km, 4 hours, 30 minutes)

• List of mountains in Seoul
• List of mountains in Korea
• Yangjaecheon

• Bayer, Achim (2023). "Water, Fire, and the Ascent Above the Mountain: Symbolism and Its Application in a Buddhist Temple on Mt. Gwan'ak (冠岳山), Seoul". Kanazawa Seiryo University Bulletin of the Humanities. 7(1): 77–91.