A low-profile slot-loaded circular polarization (CP) filtering antenna of a substrate-integrated waveguide (SIW) is proposed in this paper. It is composed of a bandpass filter with an SIW inversely coupled resonant structure, an SIW cavity feeding structure, and a surface-loaded rectangular slot SIW radiation cavity. Two L-type SIW cavity resonators surrounding the radiation cavity excite the rectangular slot through openings in the sidewalls formed by metallic vias. The bandpass filter consists of three SIW rectangular cavities in parallel. The size of the cavity and the width of the coupling port are changed to adjust the bandpass characteristics. Four rectangular slots loaded by an SIW radiation cavity are used to form a rectangular ring structure. The four rectangular slots are excited by excitation signals with phases of 0, 90, 180, and −90°. The electromagnetic wave radiated by these slots is synthesized into a circularly polarized wave in the far field. For demonstration, the prototype antenna is designed, manufactured, and measured. The center frequency of the filtering antenna is 28 GHz, the impedance broadband width is about 456 MHz (1.6%), and the gain in the working band is 4.16–5.23 dBi. The axial ratio bandwidth is 270 MHz (0.9%). The measured results are consistent with the simulation results. Owing to its good performance, the proposed filtering antenna is expected to be widely used as a novel RF sensor in various IoT devices.