In this paper, a novel type of transmitarray antenna based on 3-D frequency selective structures (FSSs) is presented to achieve high-gain, low-profile, and wideband performance. By using two groups of planar slotline structures in each element, the proposed FSS can produce a fourth-order bandpass filtering response with more than 360 degrees phase shifts in the passband. Then, to meet the requirement of phase compensations for the transmitarray, twelve 3-D FSS elements with different dimensions are carefully designed to achieve the desired transmission phase shifts in the operation band. Based on these 3-D FSS elements, a transmitarray antenna is designed and simulated with the center frequency of 8 GHz. The profile of the transmitarray without the feeding antenna is 0.22\lambda 0 where \lambda 0 is the free-space wavelength at the center frequency. The simulated gain of the designed transmitarray antenna is 22.91 dB at around 8 GHz, resulting in 39.35% aperture efficiency. The simulated antenna bandwidths are 9.8% for 1dB gain and 22 % for 3dB gain, respectively.