This paper presents a leaky horn antenna based on a substrate integrated waveguide (SIW) with monopulse performance. The proposed leaky horn antenna consists of two SIW leaky-wave antennas (LWAs) angularly separated to form the horn's flaring. By properly designing this angular separation taking into account the main radiation direction of both LWAs, a flat wavefront is achieved in the leaky horn aperture. To provide the monopulse behavior to the SIW leaky horn, a hybrid coupler, and phase shifters are included in the feeding. Depending on the input port chosen on the hybrid coupler, the LWAs forming the leaky horn are fed in phase or counter-phase and generate the sum and difference radiation patterns. A prototype has been fabricated to validate the design. In the operating band, an impedance matching below −13.9 dB is achieved at both input ports with an isolation between ports better than 16.6 dB. The measured radiation patterns show a monopulse behavior with a proper agreement with the simulated results. The proposed leaky horn presents a depth of −25.8 dB (difference between the sum diagram and the difference in the boresight axis), a peak realized gain of 12.9 dBi, and a half-power beamwidth of 6.4° at 27.5 GHz.