A series of 3,4-dihydro-2H-1,4-benzo[b]thiazines (34 examples) were efficiently synthesized in good to excellent yields through a 1,4-diazabibicyclo[2.2.2]octane (DABCO) catalyzed one-pot, three-component reaction of 2-aminobenzenethiol, aromatic aldehydes and α-halogenated ketones in the presence of K₂CO₃. The mechanism for this process was briefly discussed with a tentative catalytic cycle proposed.