A wideband microstrip patch antenna using an L-shaped parasitic structure was developed to address the inherent narrow bandwidth limitation of conventional microstrip antennas. The antenna consists of a main radiating patch and L-shaped parasitic patches, with an overall size of 0.83λ?×0.83λ?×0.083λ?, where λ? is the free-space wavelength at the center frequency of 5 GHz. The main patch operates in the fundamental transverse magnetic (TM10) mode. Multiple resonant modes are excited through electromagnetic coupling with the parasitic elements to broaden the bandwidth. A Rogers 5880 substrate with a relative permittivity of 2.2 and a loss tangent of 0.0009 was used to reduce the quality factor. A 180-degree out-of-phase differential feeding scheme was applied to suppress pattern distortion. Simulation results show that the antenna achieves a voltage standing wave ratio (VSWR) below 2 from 3.6 GHz to 6.6 GHz, corresponding to a 60% relative bandwidth—six times wider than that of traditional designs. A peak gain of 9.8 dBi occurs at 6.2 GHz, with gain variation less than 1.5 dB across the operational band. The radiation pattern maintains main beam deviation within 5 degrees and cross-polarization levels better than -15 dB over 3.6–6.3 GHz, reaching as low as -38 dB at the center frequency. This single-layer design achieves wideband and high-gain performance, making it suitable for broadband wireless communication systems such as fifth-generation (5G) mobile networks and Wi-Fi.