The rotational spectrum of the deuterated species of thioformaldehyde, HDCS, has been measured in the laboratory by microwave spectroscopy. Fifty-two a-type R-branch transitions are observed, and the rotational constants and centrifugal distortion constants are determined accurately. On the basis of the laboratory results, two unidentified lines detected during the line survey toward a cold dark cloud TMC-1 are assigned to HDCS. The [HDCS]/[H2CS] ratio is determined to be 0.020 +/- 0.005, indicating relatively high deuterium fractionation in H2CS. The ratio is discussed in terms of formation processes of H2CS in dark clouds.