A high-gain square patch antenna using the TM20and TM21 resonant modes to realize reconfigurable bidirectional and broadside beams is proposed in this paper. Initially, two open slots are loaded to the edges of the radiating patch in order to manipulate the on-patch current distribution, by which the radiation pattern of TM21 mode is transformed from four beams to the desired broadside beam while that of TM20 mode is still bidirectional. With the help of slot loading, the resonant frequency of TM21 mode can additionally get reduced towards that of TM20 mode. After that, the two modes can be independently excited via a simple feeding network with common-mode and differential feeding schemes, respectively, bringing out switchable broadside and bidirectional patterns the measured results are found in reasonable agreement with simulation, demonstrating that the antenna can achieve an overlapping bandwidth of 7.3%, and realized gains as high as 10.4 and 9.6 dBi for the broadside and bidirectional states, respectively.