In this paper, a compact microstrip patch antenna (MPA) with enhanced bandwidth and filtering response is proposed and implemented solely by loading pairs of slots on the conventional square patch radiator. Initially, the dominant mode TM10 of the square patch radiator is theoretically investigated, revealing its broadside radiation pattern and its current flows in the consistent direction. After that, based on the regularities of field distribution, two pairs of slots are etched in the square patch so as to generate two extra modes without destroying both resonant and radiating properties of TM10 mode. By virtue of these two excited modes, two extra resonances are produced in proximity to its dominant one, so as to widen the bandwidth under triple resonant modes. Meanwhile, owing to the inverse equivalent magnetic currents (EMCs) along the radiation edges of the patch and the out-of-phase electric fields at the two sides of each slot under these two excited modes, radiation cancellation towards broadside direction and efficiency reduction occur simultaneously. Hence, radiation nulls are achieved in all direction and particularly the radiation null towards broadside direction are deeper than others. After our intensive investigation is done on the entire slots-loaded patch radiator, a wideband filtering MPA with three in-band resonances and two out-of-band radiation nulls has been finally constructed. For demonstration, a prototype antenna at a center frequency of 3.5 GHz is designed, fabricated and measured. The measured results are found in good accordance with the simulated ones, demonstrating an impedance bandwidth of 9.14% with stable broadside realized gains as well as two radiation nulls at 3.12 and 3.83 GHz, respectively, in addition to its low profile of 0.029 free-space wavelength. Notably, the presented antenna has occupied the almost same area as the conventional square MPA, maintaining its good size compactness.