This communication presents a Ka-band multibeam substrate-integrated waveguide (SIW) horn array implemented in a single-layer planar substrate. By conforming the array into a circular shape, the horn array covers full 360° azimuth range with 12 beams. Every adjacent beam overlaps each other at the half-power beam shoulders. A wideband matching is obtained with -10 dB reflection coefficients from 27.5 to 38 GHz. By suppressing the side-lobe levels to be lower than -20 dB, the mutual coupling between each SIW horn is less than -45 dB from 28 to 38 GHz. With a thin radiating aperture of 0.1 $6\lambda$ , a stable realized gain of each beam is achieved around 9.5 dBi, whereas it increases to 12.9 dBi when there is a conducting reflector closely beneath it. Due to the reflector, the main radiation directions tilt 30°, which enables the proposed multibeam antenna to be mounted on the ceiling while maintaining the communication for multiple users below. Results with a manufactured prototype well demonstrate the multibeam performance both in simulation and measurement with good agreement. In addition, it is compact and easy to be fabricated with common printed circuit board (PCB) techniques, also suitable to be integrated into microwave systems. It is promising for the application in beam-switched, multiple-input multiple-output (MIMO), and omnidirectional systems.