We describe the design, construction, and performance of a waveguide Orthomode Transducer (OMT) for the 385-500 GHz band. The OMT is based on a symmetric backward coupling structure with a square waveguide input (0.56x0.56 mm2) and two single-mode waveguide outputs: a standard WR2.2 waveguide (0.56x0.28 mm2) and an oval waveguide with full-radius corners. The OMT is rescaled from a lower frequency design that was developed for the 3 mm band; it was optimized using a commercial 3D electromagnetic simulator. The OMT consists of two mechanical blocks in split-block configuration, fabricated using a CNC micromilling machine. A first prototype copper alloy OMT employing standard UG387 flanges at all ports was fabricated and tested. From 385 to 500 GHz the measured input reflection coefficient was less than -10 dB, the isolation between the outputs was less than -25 dB, the cross polarization was less than -10 dB, and the transmission was ≈-2 dB at room temperature for both polarization channels. The effects of misalignment errors in the OMT performance were studied using electromagnetic simulation. A second OMT version utilizing custom made mini-flanges and much shorter waveguides was designed and will be tested soon. This novel OMT is more tolerant to misalignment errors of the block halves and is expected to have much improved performance over the first prototype.