In this article, a high-gain square patch antenna with reconfigurable broadside and bidirectional beams under operation of TM20 and TM21 modes is proposed. Initially, two open slots are loaded to the edges in two opposite sides of the radiating patch so as to reshape the current distribution of TM21 mode, by which its four-beam radiation pattern is transformed into the desired broadside beam. With this arrangement, the resonant frequency of TM21 mode can additionally get reduced towards that of TM20 mode. By utilizing a simple feeding network with four p-i-n diode switchers, TM20 and TM21 modes can be further excited with common-mode and differential feeding schemes, respectively, thus bringing out switchable broadside and bidirectional patterns. The simulated and measured results are found in good agreement, both demonstrating that the antenna achieves an overlapping bandwidth of 7.3% in a range from 2.35 to 2.54 GHz, and the measured peak gains as high as 10.4 and 9.6 dBi for the broadside and bidirectional states, respectively.