A novel series-fed microstrip patch array antenna for 37/39 GHz beamforming is proposed. To improve the antenna bandwidth, two of the patches are modified with truncated corners in the diagonal direction. This truncation generates two degenerate resonances which result in a flattened frequency response of the input impedance. Then, the recessed microstrip feeds for the other two patches are designed to yield a proper current distribution for radiation while maintaining minimal return loss, wide bandwidth, and low sidelobes. Though the individual patch antenna is elliptically polarized due to the truncated corners, a phased array with linear polarization can still be obtained by alternately deploying left-handed and right-handed elliptically polarized patches. For validation of the proposed design, an array is fabricated with 16 elements on a substrate with 10 mil thickness and ϵ r =2.2. The beamforming capability of the proposed array is also demonstrated. The experiment results agree well with the simulation and show that the antenna gain and the return loss bandwidth can be more than 21 dBi and 8%, respectively.