The National Championship was the next stage from the regional stage of 2021–22 Thai League 3.
The winners and runners-ups of each region would qualify for this round to find 3 clubs promoting to 2022–23 Thai League 2.
Winners, runners-up, and third place of 2021–22 Thai League 3 would be promoted to the 2022–23 Thai League 2.
Nakhon Si United won 4–3 on aggregate.
Uthai Thani won 3–1 on aggregate.