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.
Relegated to K4 League Promoted from K4 League Withdrawn
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.
Gimpo FC won 3–2 on aggregate.