Methane sulfonamide derivatives of 3‐amino‐2‐thioxo‐2,3‐dihydrothieno2,3‐d]pyimidin‐4(1H)‐one, potential selective COX‐2 inhibitors, were synthesized and their structural elucidation is here reported. Some derivatives, at 10 μM concentration, showed a significant percentage of inhibition in some in vitro experiments.