The complex thiirane–trifluoromethane (C2H4S⋯HCF3) has been observed in a supersonic expansion using Fourier-transform microwave spectroscopy. Its structure was derived from the rotational spectra of the parent and three minor 34S and 13C isotopomers observed in natural abundance. The thiirane–trifluoromethane dimer exhibits an equilibrium Cs symmetry stabilized by one C–H⋯S and two C–H⋯F–C weak hydrogen bonds. The rotational spectrum shows A–E splittings due to the internal rotation of trifluoromethane in the complex around an axis close to its C–H bond, suggesting that the C–H⋯S linkage is the primary hydrogen bond interaction in the dimer.