Mongchontoseong (Korean: 서울 몽촌토성) is an ancient earthen rampart dating from the Baekje period. It is located what is now in the Olympic Park of Seoul, South Korea.

The fortification walls are estimated to have been about 2.7 km (1.7 mi) in length and approximately 6 to 7 m (20 to 23 ft) high. The fortifications of Mongchontoseong had a palisade atop the wall and a moat surrounding its base. They are part of Wiryeseong with Pungnaptoseong.

During the 1988 Summer Olympics, the running section of the modern pentathlon was held there. A number of important excavations of the site were conducted prior to the construction of the nearby Olympic Park.

• 1988 Summer Olympics official report. Volume 1. Part 1. p. 181.

• Mongchontoseong Earthen Fortification, Seoul at Cultural Heritage Administration of the Republic of Korea