This paper presents a broadband dual-polarized substrate integrated waveguide (SIW) slot antenna. It is analytically shown that by decreasing the permittivity of a dielectricloaded slot antenna, the resulting bandwidth increases significantly, where the widest bandwidth can be achieved when the permittivity of the dielectric approaches near zero. To demonstrate this idea, a rectangular SIW slot is loaded by arrays of thin wires to realize epsilon-near-zero (ENZ) metamaterials (MTM), which consequently resulted in a simple, single-layer, compact and cost-effective structure. The measured impedance bandwidth (|S11| < -10 dB) of 13.6%, covering the millimetre wave (mmWave) frequency range of 22.6 GHz to 25.9 GHz is achieved. The radiation efficiency is higher than 80%, providing at least 9.5 dBi gain through the entire frequency band.