We report the measured sensitivities of a superconducting NbN hot electron bolometer (HEB) heterodyne receiver at 5.25 THz. Terahertz (THz) radiation is quasioptically coupled to a HEB mixer with a lens and a spiral antenna. Using a measurement setup with black body calibration sources and a beam splitter in vacuo, and an antireflection coated Si lens, we obtained a double sideband (DSB) receiver noise temperature (TrecDSB) of 1150 K, which is nine times hν/2k, where h is the Planck constant, ν the frequency, and k the Boltzmann constant. In addition, the measured far field beam patterns of the integrated lens antenna show nearly collimated beams from 2.5 to 5.3 THz that allow reliable measurement of TrecDSB using the vacuum setup. Our experimental results in combination with an antenna-to-bolometer coupling simulation suggest that the HEB mixer can work well at least up to 6 THz, making it suitable for next generation of high-resolution spectroscopic space telescopes and, in particular, for the detection of the neutral atomic oxygen line at 4.7 THz.