In this paper the gain enhancement of an ultra-wideband (UWB) antenna, achieved using an appropriately designed multi-layer frequency selective surface (FSS) reflector is demonstrated. The proposed novel FSS reflects effectively in-phase over a bandwidth of about 145%. Consequently, significant enhancement in antenna gain has been achieved with a low-profile configuration without compromising the impedance bandwidth of the UWB antenna. The composite structure is compact, with a total height of λ/4 where λ is the free-space wavelength at the lowest operating frequency of 3 GHz. Experimental results show an impedance bandwidth of 145% BW with FSS and 149% without FSS. The antenna gain is maintained around 9.3 dBi from 3 to 15 GHz with a variation of ±0.5 dB. The predicted wideband antenna performance and gain enhancement due to the presence of the FSS reflector are discussed.