A compact substrate integrated waveguide (SIW) cavity slot antenna with enhanced bandwidth and quasi-elliptic filtering response is proposed. Served as radiating elements, two slots are etched on the top plane of a square SIW cavity. Quality factors of two original modes are greatly reduced and a new resonant mode is introduced by etching an additional slot on the bottom plane. By merging these three modes within a single passband, the bandwidth can be enhanced. In addition, a radiation null is generated in the lower and upper stopband, respectively. Therefore, a quasi-elliptic filtering response is achieved with three resonant modes in the passband and two radiation nulls in the stopband. A fabricated prototype with the size of 0.48λ 0 × 0.48λ 0 × 0.02λ 0 has been measured. The measured 10 dB bandwidth is 11.5% (3.28–3.68 GHz). The measured efficiency is about 77% in the passband but it sharply drops to 5.6% at 3.26 GHz and 10.5% at 3.76 GHz.