2020–21 Thai League 3 National Championship

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.

Teams
Note:

Play-off round


!colspan=3|Northern region

!colspan=3|Bangkok metropolitan region


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 * Northern region


 * Bangkok metropolitan region

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

Summary



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Matches
Rajpracha won 2–1 on aggregate.

Summary



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2nd leg
Lamphun Warriors won 3–2 on aggregate.

Teams promoted to 2021–22 Thai League 2

 * Lamphun Warriors (champions)
 * Muangkan United (runners-up)
 * Rajpracha (third-placed)