We present the first gas phase spectra of singly deuterated oxadisulfane, HSOD, in its vibrational ground state. More than 100 transitions have been recorded with highest frequency accuracy using the Cologne Terahertz Spectrometer. The molecular parameters derived from a least squares fit analysis proof HSOD to be an almost accidental symmetric prolate top molecule with an asymmetry parameter kZK0.9985. Spectra of c-type and weaker b-type transitions have been recorded in the range from 716 to 772 GHz. The ratio of the dipole moments m c /m b Z2.4(3) has been derived from measured line intensities. The c-type transitions are split by the tunneling motion of a hindered internal rotation, whereas b-type transitions show no splitting within the Doppler limited line profiles. We derived the equilibrium molecular structure of oxadisulfane, HSOH, from experimental values of the rotational constants A 0 , B 0 , and C 0 of HSOH, H 34 SOH, DSOD, and HSOD. The equilibrium rotational constants A e , B e , and C e were derived by taking vibration-rotation interaction constants a r obtained from high-level ab initio calculations into account.