Quantum Cascade Lasers (QCL) are the most promising technology for producing compact, high power (> 1 mW), coherent signal sources above 2 THz. Due to their small size (10 μm × 25 μm) and rectangular cross-section, the output beam from a QCL laser cavity is highly divergent and non-Gaussian. A single mode Gaussian beam is desirable for efficient coupling to optical systems. We have designed a waveguide spatial filter for this purpose. The 2.7 THz spatial filter consists of two diagonal feed horns connected by one wavelength of square waveguide (92 μm on a side). The mode filtering efficiency and far field beam pattern of the structure have been modeled in CST Microwave Studio. We have fabricated the filter in tellurium copper using a Kern MMP micromilling machine. We present measurements of the QCL's throughput and emergent power pattern with and without the filter. Our preliminary findings suggest that spatial filtering significantly improves the QCL beam pattern, and further measurements are being made to more rigorously explore these results.