Ansan (Hangeul: 안산, Korean pronunciation: ) is a city in Gyeonggi Province, South Korea.  It lies southwest of Seoul and is part of the Seoul National Capital Area. It is connected to Seoul by rail via Seoul Subway Line 4. It is situated on the Yellow Sea coast and some islands lie within its jurisdiction. The largest and best-known of these is Daebu Island.
 Several higher learning institutions are located in Ansan.  They include Ansan University, Shin Ansan University, Seoul Institute of the Arts, and the ERICA campus of Hanyang University. The Korea Transportation Safety Authority, a government agency, has also been headquartered in Ansan since June 3, 2002.  With its high number of foreign workers, Wongokbon-dong has been designated as a multicultural area. In 2021, Ansan was selected as the largest residential area for foreigners and it is seeking designation as a special multicultural city. Street play festival is held every year in Ansan.
 The first humans in Ansan were in the New Stone Age, and many shell middens and prehistoric remains were found at Oido, Sihwaho, Chojidong and Daebudo. In the Seonbu-dong and Wolpi-dong area, over 10 stone dolmen tombs could be found.: 28  Also in 1995, Old Stone Age relics were found while constructing the Seohaean Expressway. Most dolmens in Ansan are north dolmen, but the dolmen in Seonbu-dong is table-shaped and another table-shaped tomb was found in Hakon-dong, Gwangmyeong. There are many ancient relics found in the city.
 Not much evidence or antiquities survive from the Proto-Three Kingdoms and Samhan periods are in Ansan and nearby areas.: 29  The Mahan confederacy in Chen Shou in Records of the Three Kingdoms preserves writings about the chiefdom state. Ansan was in communion with Baekje but very little has been saved.
 In the Later Three Kingdoms era, the Ansan area was owned by Taebong (also known as Hugoguryeo, "Later Goguryeo") in Silla Hyogong's 4th year (AD 900).: 31  After then, Gung Ye, who was the ruler of Taebong, was overthrown by Wang Geon, and the Ansan area was turned over to Goryeo, founded by Wang Geon. First, the Gung Ye Ganggu-gun was changed to Ansan-gun but the state of the military was preserved.
 Ansan belonged to Gyeonggijwa-do (left Gyeonggi province) in the Joseon dynasty. Taejo's 5th year (AD 1396), Ansan-eoso (meaning Ansan fishery) that is now under the direct control of a detached building of Saongwon (the Department of foods and meals in the Joseon dynasty) was set up.: 32  After Gyeonggijwa-do and Gyeonggiwo-do (right Gyeonggi province) were unified in Teajong's 2nd year (AD 1402), Ansan was still called Ansan. Under Teajong's 13th (1413), the whole country was detached into eight provinces.
 Until 1914, Ansan City and the southern part of today's Siheung city consisted of Ansan County. In 1914, Ansan County was annexed to Siheung County. This region produced high-quality salt from ancient times and valuable marine products. Ansan was then a fishing village.
In 1986, several towns of Hwaseong county became incorporated, representing the beginning of Ansan as a city. With currently over 700,000 residents, Ansan now has two distinct and separate districts: Danwon, which has 12 dongs as of 2009, and Sangnok, with 13.
 Ansan is located in the southwest of Gyeonggi Province and is situated on the coast of the Yellow Sea (locally called West Sea), at 37°19′N 126°50′E﻿ / ﻿37.317°N 126.833°E﻿ / 37.317; 126.833. Portions of various Yellow Sea islands lie within its jurisdiction. The largest and best-known of these is Daebu Island, also known as "Daebudo".
 Located at the south of the city (but within its jurisdiction) are islands of Daebudo and Pungdo. On land, the city boundaries is marked by the town of Gunpo to the east of Ansan, Anyang to north-east, Siheung to the north and Hwaseong to the south.
 Ansan has a monsoon-influenced humid continental climate (Köppen: Dwa) with cold, dry winters and hot, rainy summers.
 The Ansan Line is a major rail line in Ansan and is part of the Seoul Metropolitan Subway as Line 4. Service connects from Oido to Jinjeop in Namyangju. The subway is also connected to the public bus system in Ansan. Previously, the first railroad in the city was the Suin Line which was opened by Korea under Japanese rule.
In mid 2020, the Suin Bundang line started operations, connecting Suwon and Ansan which expanded the living space in Ansan.
 Currently, Seohae Line starts from Ansan to Sosa Station in Bucheon. But the Ministry of Land, Infrastructure and Transport is extended to Goyang through Gimpo International Airport Station. It was planned to be completed by 2023 and extended Seohae Line officially opened on 1 July 2023.
 The Shin Ansan Line will connect Yeouido to Ansan in 2024. It will take only 30 minutes from Yeouido to Ansan Also, the Ministry of Land, Infrastructure and Transport is considering to extend to Seoul Station.
 The Ansan Bus Terminal (안산종합버스터미널) near Ansan Station provides daily bus service to Incheon International Airport and most cities in South Korea including Gimhae-si, Busan-si. Buses travel via Gimpo International Airport and Incheon International Airport. Express buses to Iksan, Dongdaegu, and Gwangju are also available. Near Sangnoksu Station are buses going to Suwon Station and Gangnam Station.  Ansan has 53 bus lines with 537 buses operating in and out of the city limits, connecting the city with Seoul and other outlying Gyeonggi cities. The Ansan transfer center also has express city buses, city buses, and intercity buses to link to Banwol industrial estate, Sihwa lake, and nearby cities.
 Pedalro is a public bicycle system in Ansan City. There are 101 Pedalro stations in Ansan. This is run on a membership basis. However, due to high cost for maintenance, Ansan city discontinue the public bicycle service in 2021.
 The Korea Transportation Safety Authority, a government agency in South Korea, is headquartered in Ansan. Other major institutions in Ansan include:
 The administrative district of Ansan is parted into 25 Dong, 1187 Tong, and 5884 Ban. The extent of Ansan is 149.06 km2 (Sangnok-gu: 57.83 km2 Danwon-gu: 91.23 km2). The population standard was 281,829 households and 761,279 people at the end of 2012, of which the Korean citizen population was 715,108 with 43,465 foreigners recorded.
 Ansan had Hyanggyo, nationally sponsored schools, throughout the Joseon Dynasty and Goryeo. Modern institutions of higher learning located in Ansan include Ansan University, Shin Ansan University, Seoul Institute of the Arts, and the ERICA campus of Hanyang University.
 List of universities in Ansan:
 There are 24 high schools, 29 middle schools, 54 elementary schools and 96 kindergartens in the city. Danwon High School in Ansan was particularly affected by the Sinking of the MV Sewol in 2014 as a large number of the passengers were students from the school, and city streets featured many ribbon memorials and other markers and ceremonies for the disaster. Ansan Seo Elementary School a.k.a. Ansan West Elementary School (안산서초등학교) in Wongok-dong has a multicultural program with students from other countries, a rarity in South Korean education.
 Because Ansan is a planned city built behind the Ansan Smart Hub following the government's industrial planning, the proportion of agriculture in the Ansan economy is very low. Meanwhile, as the government promoted a manufacturing industry plan in the Ansan area before Ansan was built, many people work in the manufacturing industry. There are two national industrial complexes, Banwol and Sihwa, as well as the local Banwol plating industrial complex.
 In the past, Ansan had fewer attractions and a poor reputation due to its industrial and fishing roots, but the east side of the city has recently experienced several beautification projects. The canal systems which bisect the city have bicycling and walking paths, and are covered with roses, tulips, and cherry blossoms in spring. A community bicycle system sponsored by the city is also available for park access and riding. Nowadays, Ansan has many museums and galleries, such as the Ansan Culture and Arts Center, Seongho Memorial Hall, Danwon Sculpture Park, and Hwarang Public Garden. Ansan has run city bus tours for cultural heritage sightseeing since 2008.
 The Jet-mer-ri village deity is held every year in the Jet-mer-ri shrine to the village deity from 1984, and since 1987 the Beal-Mang-Sung art festival is held. Also, since 1991 the Danwon art festival and since 1996 the Sung-ho cultural festival is held. The Ansan Street Arts Festival is held in Ansan Culture Square in May since 2005.
Other festivals in Ansan include:
 Historical monuments and structures in Ansan include:
 Modern monuments and cultural institutions in Ansan include:
 Ansan is home to several parks, including:
 Large and small stadiums have been founded from the 1990s on in Ansan. Major sports events in the city take place at the Ansan Wa~ Stadium.
 The city was home to Shinhan Bank S-Bird Woman's Basketball Team (member of the Women's Korean Basketball League). The team moved to Incheon in 2014. It was also a home to the now-defunct National League soccer club Ansan Hallelujah.
 Ansan is also home to Ansan OK Financial Group Okman volleyball club. They are members of the V-League.
 Ansan established a professional football team called the Ansan Greeners FC. In first year, the team also received the 'Plus Stadium' award and the "Full Stadium" award. At present (April 10, 2018), the Ansan Grinders FC has placed second.
 The pension complex on Daebu Island is used as a location for dramas such as Boys Over Flowers and A Man Called God.