This paper reports the design, manufacture and characterisation of a new frequency selective surface (FSS) structure which meets the demanding requirements for transmission of 50.2 − 57.7 GHz radiation simultaneously for TE and TM polarizations at 45° incidence, and reflection of signals in four discrete higher frequency bands centered at 89 GHz, 165.5 GHz, 183.3 GHz and 229 GHz. The FSS is required for a quasi-optical network, which was developed during preparatory breadboarding of the Microwave Sounder instrument. The 100 mm diameter ultra-wide band FSS must exhibit ≤0.25 dB loss for all signals in the above bands, and has to satisfy the requirements of the space environment. The FSS is formed by a periodic metal film array sandwiched between two 0.83 mm thick, optically flat, fused quartz substrates. It has 19,000 unit cells composed of two compact resonant slot elements, a meandering elliptical annulus and a folded dipole. Spectral transmission and reflection measurements in the 50 − 230 GHz frequency range yielded results that are in excellent agreement with numerical predictions.