2021 K4 League

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.

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.

Team changes
Relegated from K3 League
 * Chuncheon Citizen
 * Jeonju Citizen

Newly joined
 * Dangjin Citizen
 * Gangwon FC B
 * Geoje Citizen
 * Pyeongchang United

Promoted to K3 League
 * Paju Citizen
 * Ulsan Citizen

Withdrawn
 * Icheon Citizen

Promotion play-off
When the match was finished as a draw, its winners were decided on the regular season rankings without extra time and penalty shoot-out.