The ground-state rotational spectrum of the tetrahydrothiophene ... water complex (C4H8S ... H2O) has been studied by free jet millimeter wave absorption and molecular beam Fourier transform microwave spectroscopy. The spectra of H2O and D2O combined with (C4H8S)-S-32 and (C4H8S)-S-34 were assigned. The rotational parameters have been interpreted in terms of a geometry in which the water molecule acts as proton donor lying close to the plane bisector to the CSC angle of tetrahydrothiophene. The "free" hydrogen is entgegen to the ring. The parameters characterizing the hydrogen bond are the distance between the sulfur and hydrogen atoms, r(S ... H) = 2.37(4) Angstrom, and the angle between the line bisecting the CSC angle of tetrahydrothiophene and the S ... H bond, phi = 85.(3)degrees. The deviation from collinearity of the atoms S ... H-O is suggested from theta = 162.(12)degrees.