The 2021–22 Thai League 3 is the fifth season of the Thai League 3, the third-tier professional league for association football clubs in Thailand, since its establishment in 2017, also known as Blue Dragon League due to the sponsorship deal with Blue Dragon Lottery Online.

1st and 2nd places of each zone qualified for this stage by being featured in 2 groups.

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

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

Winners, runners-ups, and third-placed of the national championship stage would be promoted to the 2022–23 Thai League 2.