We propose a dual-polarized leaky-wave antenna (LWA) with orthomode based on a metallic square waveguide for broadband endfire radiation applications. Several double-sided slots acting as radiation elements are notched on the waveguide to produce an endfire pattern. To overcome the restriction associated with the endfire radiation condition of a linear array, a metallic core periodically loaded with pins is used to control the dispersion of the proposed double-sided-slot waveguide. By optimizing the height distribution of the loading pins along the longitudinal direction using a cosine function, stable endfire radiation can be achieved over a broad operating band. Moreover, dual-polarized radiation can be automatically implemented because of the rotational symmetry of the proposed LWA. Furthermore, two rectangular waveguides connected with a square waveguide are used as the orthomode transducer (OMT) of the proposed LWA. The numerical and experimental results verify that dual-polarized endfire radiation is achieved in the frequency range of 7.6–9.1 GHz with an isolation of higher than 45 dB. The realized gain achieved is in the range of 13–15 dBi with a cross-polarization level of below −30 dB.