This paper presents a display-integrated optically invisible phased array using the antenna-on-display (AoD) technology for millimeter-wave 5G cellular devices. Based on the proposed approach, a massive number of antennas can be embedded in organic light emitting diodes (OLED) or liquid crystal displays (LCD) of cellular devices, while consuming near-zero volume. To obtain simultaneously high optical transparency and electrical conductivity, diamond-grid shaped transparent electrodes are developed and analyzed. The designed and fabricated antenna element exhibits an impedance bandwidth of 3.62 GHz with a center frequency of 26 GHz and a measured realized gain of 1.78 dBi. The designed antenna element is extended to the 1×4 phased array which is assembled with the 5G beamformer board to verify the beamforming capabilities. Measured radiation patterns confirm that the proposed phased array module achieves a beam-steering range of ± 30° in the azimuth with a maximum EIRP of 18.8 dBm at 28 GHz.