In order to enhance the scanning range of planar phased arrays, a planar substrate integrated waveguide slot (SIW-slot) antenna with wide beamwidth is proposed in this letter. The proposed antenna is fabricated on a single-layer substrate, which is fully covered with a metal ground on the back. The SIW works like a dielectric-filled rectangular waveguide working in the TE10 mode. There are four inclined slots etched on the top metal layer following the rules of rectangular waveguide slot antenna. The electric fields in the slots work as equivalent magnetic currents. As opposed to normal microstrip antennas, the equivalent magnetic currents from the slots over a larger metal ground can radiate with a wide beamwidth. Its operating bandwidth is from 5.4 to 6.45 GHz with a relative bandwidth of 17.7%. Meanwhile, the 3-dB beamwidth in the xz-plane is between 130° and 148° in the whole operating band. Furthermore, the SIW-slot element is employed in a 1×8 planar phased array. The measured results show that the main lobe of phased array can obtain a wide-angle scanning from −71∘ to 73° in the whole operating band.