Onsŏng County is a county (kun) in North Hamgyong Province, North Korea, located near the border with China. The administrative center is the town (ŭp) of Onsong. Onsong is the alleged site of the former Onsong concentration camp, now closed.

Onsong was one of the six post/garrisons (Korean: 육진; Hancha: 六鎭) established under the order of Sejong the Great of Joseon (1418–1450) to safeguard his people from the hostile Chinese Ming dynasty and Manchurian/Jurchen nomads living in Manchuria.

Onsong lies along the Tumen River, which forms the border with China. To the north of Onsong is Jilin Province, China. Onsong also contains the northernmost point in Korea, at 43°0'39″ N. Liangshui (Korean: 량수진; Hancha: 凉水鎮), in Tumen prefecture, is the closest Chinese town across the river.

The land of Onsong is mainly mountainous, although a part is flatland. It has a continental climate, and is the region of Korea with the longest winter, except the Kaema Plateau. The highest peak is Yŏndubong.

Being mountainous in nature, Onsong is not well suited to agriculture. Ironically, however, it has the highest ratio of rice paddies to dry fields of any district in North Hamgyong Province. Logging is prevalent in the Tumen River basin. Lignite coal is also produced.

Onsong is served by roads and railroads. In winter, it is also possible to cross the frozen Tumen River into China.

Onsong has a 4 km long, single overhead bidirectional trolleybus line linking from the railway station to the Wangjaesan Grand Monument, although no trolleybuses are observed on the line since at least 2004. The overhead is nonetheless at least appearing to be in a good condition. The system formerly had two Jipsan 88 trolleybuses, though they were converted from the similar, but unrelated Jipsan 88 bus.

Onsŏng County is divided into 1 town ("Ŭp"), 10 workers' districts ("Rodongjagu") and 15 villages ("Ri"):

	Chosŏn'gŭl	Hancha
Onsŏng-ŭp	온성읍	穩城邑
Chongsŏng-rodongjagu	종성로동자구	鍾城勞動者區
Chuwŏl-lodongjagu	주원로동자구	周原勞動者區
Ch'angp'yŏng-rodongjagu	창평로동자구	蒼坪勞動者區
Namyang-rodongjagu	남양로동자구	南陽勞動者區
Ont'al-lodongjagu	온탄로동자구	穩炭勞動者區
P'ung'il-lodongjagu	풍인로동자구	豊仁勞動者區
Sambong-rodongjagu	삼봉로동자구	三峰勞動者區
Sanghwa-rodongjagu	상화로동자구	上和勞動煮區
Sansŏng-rodongjagu	산성로동자구	山城勞動者區
Tongp'o-rodongjagu	동포로동자구	東浦勞動者區
Chungsal-li	증산리	甑山里
Hasambong-ri	하삼봉리	下三峰里
Hyangdang-ri	향당리	香堂里
Kangal-li	강안리	江岸里
Misal-li	미산리	美山里
P'ungch'ŏl-li	풍천리	豊川里
P'unggye-ri	풍계리	豊溪里
P'ungha-ri	풍하리	豊利里
P'ungsŏ-ri	풍서리	豊西里
Ryongnam-ri	룡남리	龍南里
Sesŏl-li	세선리	世仙里
Turubong-ri	두루봉리
Un'am-ri	운암리	雲岩里
Wangjaesal-li	왕재산리
Yŏnggang-ri	영강리	永江里

• Yang Dae-su (Korean: 양대수; Hancha: 楊大樹)
• Shin Rip (Korean: 신립; Hancha: 申砬), the prefect of Onsong (Korean: 온성부사; Hancha: 穩城府使), before the Imjin War
• Jung You-ji (Korean: 정유지; Hancha: 鄭有智)
• Choi Gwan (Korean: 최관; Hancha: 崔寬)
• Go Gyung-jin (Korean: 고경진; Hancha: 高景軫)
• Hyok Kang, the author of the autobiographical This is Paradise!: My North Korean Childhood

• Location of the four forts and the six posts
• Map of Onsong
• In Korean language online encyclopedias:
  • Doosan Encyclopedia (Naver)
  • Korean language Britannica (Empas) (Map)
  • Encyclopedia of Korean Culture (Empas)
  • Pascal World Encyclopedia (Nate)

42°57′20″N 129°59′30″E / 42.95556°N 129.99167°E / 42.95556; 129.99167