A wideband transmitarray (TA) antenna using compact 2-bit filtering unit cells is presented with the center frequency designed at 10 GHz. The presented TA antenna features wide bandwidth with a compact aperture size, filtering radiation performance, and low cross-polarization levels. A novel compact 2-bit phasing scheme is first proposed and illustrated by utilizing the resonating characteristics of resonators. The charging and discharging principle of a resonator is utilized to realize a 180° phase shift, while J-invertor originating from the coupling between resonators is utilized to realize a 90° phase shift. Based on the proposed phasing scheme, novel 2-bit compact filtering unit cells are designed with a compact four-copper-layer configuration. Finally, a high-gain TA antenna was designed, fabricated, and measured for radiation performance verification. Due to the elaborately designed unit cells and the cutoff effect of the feed horn, a filtering radiation performance with flat in-band gain and high out-of-band suppressions is obtained. Measured results show that a flat 3-dB gain bandwidth of 21.3% is achieved with a thin thickness of 0.11λ0 . High suppressions of 34.3 and 30.1 dB are measured over the lower and upper out-of-bands. In addition, a low cross-polarization level of −38.5 dB is obtained for high-quality wireless communications. 10.13039/501100000266-Engineering and Physical Sciences Research Council (Grant Number: EP/S005625/1).