While many transition metal (TM) nitrides—including TiN, ZrN, and TaN—have been widely studied and are currently used as hard wear-resistant coatings, diffusion barriers, and optical coatings, little is known about a related TM nitride, HfN. Here, we report the results of a systematic investigation of the growth and physical properties of HfNx layers, with 0.80⩽x⩽1.50, deposited on MgO(001) by ultrahigh vacuum reactive magnetron sputtering at 650 °C in mixed N2/Ar discharges. HfNx layers with 0.80⩽x⩽1.20 crystallize in the B1–NaCl structure with a cube-on-cube epitaxial relationship to the MgO(001) substrate, while films with 1.24⩽x⩽1.50 contain a N-rich second phase. The relaxed bulk lattice parameter of HfNx(001) decreases only slightly with increasing N/Hf ratio, ranging from 0.4543 nm with x=0.80 to 0.4517 nm with x=1.20. The room-temperature resistivity ρ of stoichiometric HfN(001) is 14.2 μΩ cm and ρ(x) increases with both increasing and decreasing x to 140 μΩ cm with x=0.80 and 26.4 μΩ cm with x=1.20. The hardness H and elastic modulus E - of HfN(001) are 25.2 and 450 GPa, respectively. H(x) initially increases for both over- and understoichiometric layers due to defect-induced hardening, while E(x) remains essentially constant. Single-phase HfNx(001) is metallic with a positive temperature coefficient of resistivity (TCR) between 50 and 300 K and a temperature-independent carrier density. It is also superconducting with the highest critical temperature, 9.18 K, obtained for layers with x=1.00. In the two phase regime, ρ ranges from 59.8 μΩ cm with x=1.24 to 2710 μΩ cm with x=1.50. TCR becomes positive with x⩾1.38, no superconducting transition is observed, and both H and E decrease.