This work proposes an antenna element design aimed to mMIMO (massive multiple-input multiple-output) applications. It consists of a dual-polarized horn antenna with grooves appliance for increasing gain without compromising its profile size. Furthermore, a substrate integrated coaxial line (SICL)-based feeding network is proposed for ensuring multiple feeding with low-coupling feature, reaching coupling below -58 dB. Two feeding layers are proposed for achieving +45° and -45° at the same structure. Metallic poles are used for reducing coupling between polarizations at the same antenna element. Numerical results have shown a 3.65-GHz bandwidth centered at 25.5 GHz, with gain varying from 13.8 to 14.9 dBi throughout the operating frequency band.