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In this article, a low‐profile ultra‐wideband (UWB) circularly polarized (CP) antenna array with metasurface is presented. The antenna array consists of a 2 × 2 polarization conversion metasurfaces (PCMs) and a sequential feeding network. The PCM is composed of 4 × 4 polarization conversion unit‐cells. The unit‐cell is obtained by putting an ellipse patch into a rectangular ring, connecting the ellipse and the ring with two stubs, and then rotating it by 45°. A linear polarization (LP) slot antenna with a PCM can achieve a CP antenna, which has an impedance bandwidth of about 33.8% (5.4‐7.6 GHz) of |S11| < −10 dB and a 3‐dB axial ratio (AR) bandwidth of about 24.8% (5.8‐7.3 GHz), respectively. The UWB CP characteristics can be achieved when a UWB sequential feeding network is added to the 2 × 2 PCMs. Simulated and measured results show that the proposed antenna array has an impedance bandwidth of about 117% (1.75‐6.75 GHz) of |S11| < −10 dB and a 3‐dB axial ratio (AR) bandwidth of about 105.7% (2.24‐7.35 GHz), respectively. The usable bandwidth can reach 100.3% (2.24‐6.75 GHz). Besides, the proposed antenna array achieves a higher gain compared to the traditional slot array and the peak gain of 12.5 dBic at 6 GHz. The overall size of the antenna is 0.6λ × 0.6λ × 0.03λ (λ refers to the wavelength of the lowest operating frequency in free space). The proposed antenna array can be widely applied to various wireless communication systems. [ABSTRACT FROM AUTHOR]