A Fourier phase grating mirror will be applied to split a single, coherent terahertz source beam into multi-beams, which are spatially distributed as required for the local oscillator in an array receiver. We report on the simulation, design and fabrication of a Fourier phase grating at 1.25 THz for generating 2×2 beams, and the measurements at 1.39 THz as a proof of concept study. We find that the characteristics of the measured diffraction beams are in good agreement with the model. In addition, we present the simulation and design of a grating for 4×4 beams as required for the GUSSTO’s 4.7 THz local oscillator.