This paper presents the development of a high-order mode, low-profile, broadband H-plane horn antenna utilizing substrate integrated waveguide (SIW) technology to increase its bandwidth. The design strategically modifies the antenna's hO rn dimensions and incorporates high-order modes, thereby introducing resonant frequencies over a range of bands and significantly broadening its operational bandwidth. Furthermore, enhancements in the antenna's bandwidth and radiation properties are achieved through the extension of the waveguide aperture, modification of electromagnetic boundary conditions, and integration of impedance-matching structures. The antenna demonstrates a notable performance, with a reflection coefficient below -10 dB in the 30–52 GHz range, and a relative bandwidth of 53.6%. Additionally, it maintains a consistent gain of over 10 dBi across the operating frequency band with excellent directional radiation characteristics. The proposed SIW-based high-order mode H-plane horn antenna offers high bandwidth, low profile, and ease-of-manufacturing attributes, making it suitable for millimeter-wave applications in the Ka-band.