The pure rotation spectrum of carbonyl sulfide embedded in superfluid helium nanodroplets was measured in the frequency range from 4 to 15.5 GHz. Four lines, corresponding to the J = 1-0, J = 2-1, J = 3-2, and J = 4-3 transitions, were found. The line widths of the transitions increase with increasing rotational quantum number J, which is indicative of a distribution of the effective B rotational constant. A comparison of the pure rotational spectrum with the microwave-infrared double resonance spectrum [S. Grebenev, M. Havenith, F. Madeja, J. P. Toennies, and A. F. Vilesov, J. Chem. Phys., 2000, 113(20), 9060] reveals that the double resonance measurement scheme probes predominantly rotational transitions within the vibrationally excited state.