A 3-D-printed, 0.3-THz, back-to-back, smooth-walled, horn is presented. It was made from “material” as mating symmetric halves, with internal surfaces metallized with gold plate. The junction of the back-to-back horns hosts a 1-mm2 microfluidic aperture for feeding analytes undergoing dielectric properties’ characterization of typically solvated proteins. The configuration allows for enhanced beam–sample interaction and efficient detection than would be conventionally achieved with either traditional quasi-optical (QO) or cavity resonator circuits. The horn pair was designed for 0.3–032-THz operation. It can, however, work over a wider domain of frequencies, namely, 0.2–0.32 THz, if restrictions on cross-polarization and matching are relaxed. Initial measurements of the horn pair have shown that there is an increase of 2.5 dB in the signal strength when the horn apertures are equidistant from the focusing mirrors of a defocused QO transmissometer system.