We propose a bandwidth-enhanced four-leaf-clover-shaped photomixer antenna for use in terahertz wireless communication systems. The antenna was designed with a highly resonant radiation element, a stable DC feed element with photonic bandgap structure, and two symmetrical parasitic elements electromagnetically coupled with the main radiator for bandwidth enhancement. The antenna was optimized to have a maximum antenna-efficiency bandwidth and maximum directivity on an extended hemispherical lens. The half-maximum antenna-efficiency bandwidth of this structure was approximately 16%, and it completely covered the 275-320 GHz band. The antenna produced a maximum antenna-efficiency of 18% and a maximum directivity of 25 dBi, with a 2.4 dBi variation within the frequency band of interest, thus promising for efficient and stable operation