A novel design for a beam-switching antenna using active cylindrical slot frequency selective surface (ACSFSS) is presented. The antenna system is composed of an omnidirectional monopole antenna and the ACSFSS, which employs a new technique of switching slot arrays. The ACSFSS is made up of 12 columns with 8 slots each, dividing the cylinder by 30 degrees. To steer the beam of the antenna the diodes are set OFF and ON, so that the radiation pattern of the antenna is determined by the number of OFF state columns. To estimate the general dimension of the cylindrical FSS, an equivalent metallic reflector is introduced and optimized, and then parametric studies for the unit cell dimensions are discussed. The fabricated prototype works within the WLAN band, centered around 2.45 GHz, and can agilely select either a narrow-beam or wide-beam operating mode. Simulation and measurements confirm the operation of the ACSFSS antenna, with good matching and gain observed. In particular, the narrow-beam mode -3 dB beamwidth is 47 degrees which offers enhanced angular resolution compared with other reported beam-sweeping work.