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]
| Team | Qualifying method | |
|---|---|---|
| Lamphun Warriors | Northern region champions | |
| Phitsanulok | Northern region play-off winner | |
| Udon United | Northeastern region champions | |
| Muang Loei United | Northeastern region runners-up | |
| Pluakdaeng United | Eastern region champions | |
| Chachoengsao Hi-Tek | Eastern region runners-up | |
| Muangkan United | Western region champions | |
| Rajpracha | Western region runners-up | |
| Songkhla | Southern region champions | |
| Krabi | Southern region runners-up | |
| North Bangkok University | Bangkok Metropolitan region champions | |
| Bangkok | Bangkok Metropolitan region play-off winner |
|-!colspan=3|Northern region|-!colspan=3|Bangkok metropolitan region|}
Northern region
Bangkok metropolitan region
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Winners, runners-up, and third place of 2020–21 Thai League 3 would be promoted to the 2021–22 Thai League 2.
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Rajpracha won 2–1 on aggregate.
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Zady Moise Gnenegbe
