This letter proposes an ultra‐wideband planar patch antenna array in a millimeter wave (mmW) band. The radiation element consists of a pair of symmetrical E‐shaped patches and two pairs of grounded metallic vias. A microstrip line is selected to excite the antenna element through an H‐shaped coupling slot. The entire structure is designed on two substrates, which makes it have a relatively low cost. The periodic boundaries are adopted in element simulation to achieve good agreement of performances between the element and the final array. The proposed element exhibits triple‐resonant characteristics and achieves a working bandwidth of 62%. By employing the proposed element, an 8 × 8 array with a 1‐to‐64 full‐corporate feeding network is designed, manufactured, and measured. The measured results demonstrate that the fabricated array prototype realizes an impedance bandwidth of 71% from 22 to 46.3 GHz covering all 5G mmW bands in the K/Ka band, a gain from 18.2 to 23.4 dBi over the whole operating bandwidth, and cross‐polarization levels below –30 dB. With the merits of an ultra‐wideband, stable radiation, and a low cost, the proposed array can be applied to long‐distance mmW communications and related applications.