The dihydrophosphoryl radical H2PO in the <(X)over tilde (2)A’> ground electronic state was studied by microwave spectroscopy using a source-modulated spectrometer combined with a free-space cell. The radical was generated by de-glow discharge in a mixture of PH3 and CO2 and spectral lines of H2PO were observed in the 140-380 GHz region. The spectral pattern, including hyperfine structure, suggests that the radical has a pyramidal structure with C-s symmetry. About 1000 spectral lines of (H2PO)-O-16 and 250 lines of (H2PO)-O-18 were measured for the fine and hyperfine structures of rotational transitions up to N=10-9. Nineteen molecular constants including the hyperfine coupling constants of the phosphorus and hydrogen nuclei were precisely determined by a least-squares fit of the observed lines. Molecular structural parameters were derived from the rotational constants : r(PO)=1.4875(4) Angstrom, r(PH)=1.4287(14) Angstrom, angle HPO=115.52(10)degrees, and angle HPH=102.56(14)degrees with three standard deviations in parentheses. The PO bond is intermediate between the normal single and double PO bonds. The spin density of unpaired electrons was ascertained from the hyperfine coupling constants for the P and H atoms and is consistent with the molecular structure determined.