In this paper, we describe a three-dimensional Frequency Selective Surface (FSS) element which exhibits a flat bandpass behavior over a wide frequency band, as well as controllable low-pass angular response. The unit cell comprises a conducting waveguide, which is open at both ends, and which contains two conjoined radiating elements at the top and bottom of the guide. The element exhibits a flat bandpass response over the frequency range 12.5 GHz to 27 GHz, i.e. a −3dB bandwidth (BW/f0) of 73.4%. Additionally, we show that the angular response of the FSS can be controlled by changing its periodicity.