Microwave spectrum of the HCCP radical was detected for the first time in the X (3) Sigma(-) ground electronic state using a source-modulated microwave spectrometer. In total, 24 rotational transitions of HCCP in the 90-360 GHz region, 9 rotational transitions of DCCP in the 260-360 GHz range, and 24 rotational transitions of (HCCP)-C-13-C-13 between 130-360 GHz were measured. Hyperfine structure pertaining to the phosphorus and hydrogen nuclei was observed for HCCP, and in the case of (HCCP)-C-13-C-13, only for phosphorus. The corresponding hyperfine coupling constants were ascertained in addition to the rotational, centrifugal distortion, and fine structure constants by a least-squares analysis of the measured frequencies. From the hyperfine coupling constants determined, the spin density of unpaired electrons was estimated to be 76% for the phosphorus atom and 42% for the carbon adjacent to the hydrogen. The r(0) structure of HCCP was established from the rotational constants of HCCP and its isotopically substituted species : r(0)(CP) = 1.685 Angstrom, r(0)(CC) = 1.241 Angstrom, and r(0)(CH) = 1.057 Angstrom. These structural features are consistent with a linear phospho-allenic form that has been somewhat modified by a phosphorene.