A square patch antenna operating under TM 03 mode is proposed in this paper to realize gain enhancement, sidelobe reduction, and adjustable beamwidth by reshaping the surface current distribution on the patch resonator. On the one hand, by introducing transverse slots along the central line of patch, the medial part of out-of-phase longitudinal current is interrupted to circulate with little radiation. In this way, the patch serves as a full-wave dipole rather than a <inline-formula> <tex-math notation="LaTeX">3\lambda </tex-math></inline-formula>/2 one, and thus, the sidelobe level (SLL) in the E-plane is substantially reduced. On the other hand, the slots are installed in proximity to the nonradiative edges and at the patch center, respectively, to arouse distinctive effect on the current densities across the width of patch. Consequently, the beamwidth in the H-plane can be freely adjusted by different configurations of these slots. At last, a square patch antenna with three cascaded slots loaded along the central line is designed, fabricated, and tested. The simulated and measured results show that the directivity of a single proposed patch antenna can achieve as high as 13.3 dB with the SLL reduced to about −20 dB.