Thioformaldehyde is the simplest thial or thio aldehyde (an organic compound containing a formyl group and analogous to aldehydes, with CS replacing CO). It is unstable under usual laboratory conditions, but forms a stable cyclic trimer (H2CS)3, 1,3,5-Trithiane. H2CS has been detected in the interstellar medium and in comets.
The 211–212 transition of thioformaldehyde was observed in absorption in the direction of the Galactic Center source Sagittarius B2 by Sinclair et al. in 1973. It was later also detected also in comets by Woodney et al. (1997).