By using the low loss element design method, a wideband high-efficiency transmitarray antenna with the linear-to-circular-polarization transformation is presented. The element is composed of a linearly polarized U-slot patch antenna and a circularly polarized shaped single-arm Archimedean spiral antenna, and has the advantages of low insertion loss and good incident angle stability over a wide frequency band. The 0.14-λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -thick 1613-element transmitarray is fabricated by the standard PCB technology, and presents a measured peak gain of 31.8 dBi with the corresponding aperture efficiency of 60.2%. The measured 1-dB gain bandwidth is 15.2% from 18.25 GHz to 21.25 GHz. Simple structure, high performance and low complexity make it an attractive candidate for the low-cost high-gain circularly polarized antenna.