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We describe a 1 THz quasioptical SIS mixer which uses a twin‐slot antenna, an antireflection‐coated silicon hyperhemispherical lens, Nb/Al‐oxide/Nb tunnel junctions, and an aluminum normal‐metal tuning circuit in a two‐junction configuration. Since the mixer operates substantially above the gap frequency of niobium (ν≳2Δ/h∼700 GHz), a normal metal is used in the tuning circuit in place of niobium to reduce the Ohmic loss. The frequency response of the device was measured using a Fourier transform spectrometer and agrees reasonably well with the theoretical prediction. At 1042 GHz, the uncorrected double‐sideband receiver noise temperature is 840 K when the physical temperature of the mixer is 2.5 K. This is the first SIS mixer which outperforms GaAs Schottky diode mixers by a large margin at 1 THz. © 1996 American Institute of Physics.