An ESR spectrometer operating at 250‐GHz frequency (1.22‐mm wavelength) and 9‐T magnetic field is described. The utilization of far‐infrared (FIR) technology greatly simplifies its design and performance. Good frequency and field stability are achieved by unique designs which also conveniently permit the magnetic field to be swept. The potential utility of FIR–ESR is illustrated with examples of spectra from polycrystalline and liquid samples. In the latter case the increased spectral sensitivity to motional dynamics is stressed. Several ways in which the FIR–ESR spectrometer may be improved are also discussed.