This paper presents a wideband leaky-wave substrate-integrated waveguide (SIW) slot antenna. The proposed structure comprises slots loaded with a low-index metamaterial (MTM). The MTM is realized using an array of thin wires. Loading the antenna with low-index material not only improves the antenna's impedance bandwidth but also improves radiation, introducing a slight variation in the main lobe direction as the frequency changes. This is a feature that is challenging to attain with conventional leaky-wave antennas. The antenna operating frequency covers the frequency range of 25.2 to 33.5 GHz, with a bandwidth of 28.3%. To address high gain demands in the mm-Wave applications, a 16-element double slotted leaky-wave antenna with a maximum gain of 14.2 dBi is designed and presented. The array structure is designed specifically for ISM and 5G wireless communication, while covering the frequency range from 22.5 to 26.3 GHz.