Terahertz absorption spectroscopy was employed to measure the ground state pure rotational transitions of the water isotopologue HD18 O. A total of 105 pure rotational transitions were observed in the 0.5–5.0 THz region with
100 kHz accuracy for the first time. The observed positions were fit to experimental accuracy using the Euler series expansion of the asymmetric-top Hamiltonian together with the literature Microwave, Far-IR and IR data in the ground state and m2 . The new measurements and predictions reported here support the analysis of astronomical observations by high-resolution spectroscopic telescopes such as SOFIA and ALMA where laboratory rest frequencies with uncertainties of 1 MHz or less are required for proper analysis of velocity resolved astrophysical data.