The rotational spectrum of a symmetrical-top radical CH3CCS in the E-2(3/2) electronic ground state has been detected by microwave spectroscopy. The CH3CCS radical is produced by discharging a gaseous mixture of CH3CCH and CS2, and 50 spectral lines with the K range from -4 to 6 are observed in the 132-261 GHz region with a source modulation millimeter-wave spectrometer. In addition, 19 low-J lines with K=0 and 1 are measured in the 11-21 GHz region with a Fourier transform microwave spectrometer. Ten molecular constants including hyperfine interaction constants are determined accurately by a least-squares method. The obtained hyperfine constants are found to be about a half of those of a related radical, CH3S, indicating that the structure of CH3CCS would be expressed in a classical resonant form; H3C-C=C-(S) over dot <->H3C=(C) over dot =C=S.