This letter presents a wideband slotted substrate integrated waveguide (SIW) antenna for millimeter-wave (mm-wave) fifth-generation (5G) mobile handset applications. The antenna is vertically polarized and has a small clearance of 3.3 mm. To realize a wide operating bandwidth, two sets of vias are employed to perform a change in SIW height. Furthermore, a pair of U-shaped slots is used in both top and bottom faces of SIW to improve impedance matching and front-to-back radiation ratio. The antenna achieves a measured -6 dB bandwidth of 35.7% (23.4-33.6 GHz) and a -10 dB bandwidth of 17.4% (26.2-31.2 GHz). The beamforming capability of the antenna array is investigated. The antenna array has good scanning performance and achieves large spatial coverage.