| Competition: | K4 League |
| Season: | 2021 |
| Dates: | 13 March – 13 November 2021 |
| Winners: | Pocheon Citizen (1st title) |
| Promoted: | Pocheon Citizen Siheung Citizen Dangjin Citizen |
| Matches: | 241 |
| Total Goals: | 769 |
| Best Player: | Jung Seung-yong[1] |
| League Topscorer: | Lee Chang-hoon (30 goals) |
| Prevseason: | 2020 |
| Nextseason: | 2022 |
The 2021 K4 League is the second season of the K4 League.
The R League teams (reserve teams of K League teams) could not have been involved in the South Korean football league system originally, but were allowed to join the K4 League since this season. They had to use seven or more under-23 players in the starting line-up. Gangwon FC became the first club to enter the K4 League with reserve team.[2]
Defending champions Paju Citizen were promoted to the K3 League alongside runners-up Ulsan Citizen, and Chuncheon Citizen and Jeonju Citizen played in the K4 League for the first time by being relegated from the K3 League. Icheon Citizen was disbanded due to financial problems, while four newly-founded teams Dangjin Citizen, Geoje Citizen, Pyeongchang United (successor to K3 League Basic team Pyeongchang FC) and Gangwon FC B joined the league.
The 16 participating teams played each other twice. The top two teams were directly promoted to the K3 League, and the third and fourth-placed teams qualified for the promotion play-off.[3]
Relegated from K3 League
Newly joined
Promoted to K3 League
Withdrawn
| Club | City | Stadium | Capacity | |
|---|---|---|---|---|
| Chuncheon Citizen | Chuncheon | Chuncheon Stadium | 25,000 | |
| Chungju Citizen | Chungju | Chungju Tangeum Stadium | 3,000 | |
| Dangjin Citizen | Dangjin | Dangjin Sports Complex | 11,718 | |
| FC Namdong | Incheon | Namdong Industrial Complex Park | 2,500 | |
| Gangwon FC B | Gangwon | Chuncheon Songam Stadium | 20,000 | |
| Geoje Citizen | Geoje | Geoje Stadium | 7,500 | |
| Goyang Citizen | Goyang | Eoulimnuri Byeolmuri Ground | 2,500 | |
| Jeonju Citizen | Jeonju | Jeonju Stadium | 30,000 | |
| Jinju Citizen | Jinju | Jinju Stadium | 20,116 | |
| Pocheon Citizen | Pocheon | Pocheon Stadium | 7,000 | |
| Pyeongchang United | Pyeongchang | Jinbu Sports Park | 300 | |
| Seoul Jungnang | Seoul | Jungnang Public Ground | 400 | |
| Seoul Nowon United | Seoul | Nowon Madeul Stadium | 446 | |
| Siheung Citizen | Siheung | Jeongwang Sports Park | 1,500 | |
| Yangpyeong FC | Yangpyeong | Yongmun Sports Park | 3,000 | |
| Yeoju FC | Yeoju | Yeoju Stadium | 16,000 |
| Club | Manager | |
|---|---|---|
| Chuncheon Citizen |  Son Hyun-jun | |
| Chungju Citizen | Gong Mun-bae | |
| Dangjin Citizen | Han Sang-min | |
| FC Namdong | Kim Jeong-jae | |
| Gangwon FC B | Lee Seul-gi | |
| Geoje Citizen | Song Jae-gyu | |
| Goyang Citizen | Kim Young-ho | |
| Jeonju Citizen | Yang Young-cheol | |
| Jinju Citizen | Choi Cheong-il | |
| Pocheon Citizen | Cho Man-geun | |
| Pyeongchang United | An Hong-min | |
| Seoul Jungnang | Kim Sang-hwa | |
| Seoul Nowon United | Lee Jeong-jae | |
| Siheung Citizen | Jeong Seon-woo | |
| Yangpyeong FC | Kim Chang-yoon | |
| Yeoju FC | Shim Bong-seop |
When the match was finished as a draw, its winners were decided on the regular season rankings without extra time and penalty shoot-out.