This contribution provides the analysis and design of a low loss, multi-layer frequency selective surface (FSS) on a fused quartz substrate for radiometer applications at sub-millimeter wave frequencies. The multi-layer FSS generates a bandpass filter response and consists of three metallic FSS layers and two fused quartz substrates. The design parameters like substrate heights and sheet impedances of the FSS layers are deduced by modeling the proposed structure as a second-order Butterworth bandpass filter (275 GHz center frequency and bandwidth of 20 GHz) with two serial half-wavelength resonators and three K-inverters. The FSS employs an inductive metal mesh with a unit cell size of 200μm constructed by a micro-machining lift-off process. Transmission measurements conducted in the 220GHz-330GHz range with a quasi-optical setup verified an in-band insertion loss lower than 0.2dB.