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In this communication, an effective method to suppress mutual coupling between two substrate-integrated waveguide (SIW) slot antenna arrays is presented. To achieve that, a decoupling feed network is used. The feed network is composed of a two-layer directional coupler that is connected to two SIW-slot antenna arrays directly to establish an indirect coupling with controlled magnitude and phase, which can effectively reduce the direct coupling generated by surface waves between the array elements. A $2 \times 4$ array of slot antenna as well as the proposed decoupling network is designed, fabricated, and measured to illustrate the effectiveness of the proposed technique. The measured and simulated results verified the good decoupling performance. The measured mutual coupling is about −35 dB at the center frequency without negatively affecting the return loss and radiation patterns. The proposed mechanism of the decoupling network is simple and successfully used in SIW slot antenna arrays.