[1] Due to the COVID-19 pandemic in Thailand, the season must be postponed to start in late 2020 and end in early 2021.

[9] The winners and runners-up of each region will advance to the national championship stage to finding 3 teams promoted to the 2021–22 Thai League 2.

Teams from Northern, Northeastern, and Eastern regions would have qualified to the upper group.

Meanwhile, teams from Western, Southern, and Bangkok Metropolitan regions would have qualified to the lower group.

Winners, runners-ups, and third-placed of the national championship stage would be promoted to the 2021–22 Thai League 2.