Pure rotational spectra of the mercury-carbonyl sulfide complex have been observed by a pulsed-nozzle Fourier-transform microwave spectrometer, which has been recently constructed. The structure of the complex is found to be T-shaped, which is the same as the rare-gas-carbonyl sulfide complexes analogous to this system. The mercury-carbon distance and the Hg-C-O angle have been determined to be 3.7075(2) angstrom and 86.98(1) deg, respectively, for Hg-202-OCS from the rotational constants. Harmonic force constants of the van der Waals modes have been derived by the centrifugal distortion constants. The electric quadrupole coupling constants for Hg-201-OCS have also been determined and the values indicate small perturbation on the electric charge distribution of the mercury atom by complex formation.