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.[1]
^The second-placed and the third-placed of the Northern region have similar points, so they must play-off to find a team to qualified to the national championship stage.
^The second-placed and the third-placed of the Bangkok Metropolitan region have similar points, so they must play-off to find a team to qualified to the national championship stage.