2021 K3 League

The 2021 K3 League was the second season of the K3 League as a semi-professional league. The 15 participating teams played each other twice, and the bottom team was relegated to the K4 League.

Team changes
Relegated to K4 League
 * Chuncheon Citizen
 * Jeonju Citizen

Promoted from K4 League
 * Paju Citizen
 * Ulsan Citizen

Withdrawn
 * Gyeongju Citizen

Championship play-offs
When the first round and semi-final matches were finished as draws, their winners were decided on the regular season rankings without extra time and the penalty shoot-out.

Final
Gimpo FC won 3–2 on aggregate.