In this paper, a Fabry-Perot cavity (FPC) antenna with a partially reflective surface (PRS) consisting of two dielectric slabs with identical thickness and permittivity to increase the gain with wide bandwidth is presented. The PRS is placed in front of a broadband U-shaped microstrip patch antenna to create an air-filled cavity between the PRS and the ground plane of the antenna structure. The configuration of the two dielectric slabs aims to create a positive phase gradient of the reflection coefficient, which strongly controls the gain bandwidth performance. The proposed PRS was first designed and analyzed using a transmission line model and then verified by a full-wave simulation. The measurement results show that the proposed FPC antenna achieves a gain improvement of up to 5 dB compared to the antenna without PRS, with a 3-dB gain bandwidth of 11.5% and a broadside peak gain of 9.88 dBi. In addition, the measured impedance bandwidth is approximately 18.8% and ranges from 4.95–6 GHz, which covers the required frequency band for vehicle-to-everything (V2X) applications.