The National Championship was the next stage from the regional stage of 2020–21 Thai League 3.

The winners and runners-ups of each region would qualified to this round to finding three clubs promoting to the 2021–22 Thai League 2.

Winners, runners-up, and third place of 2020–21 Thai League 3 would be promoted to the 2021–22 Thai League 2.

Rajpracha won 2–1 on aggregate.

Lamphun Warriors won 3–2 on aggregate.