A single-mode terahertz quantum cascade laser (THz-QCL) with a two-dimensional patch antenna array as a resonant cavity is proposed and realized. The active region of each patch antenna is sandwiched between two metal layers, exhibiting full-scale subwavelength characteristics and exciting a vertical electric quadrupole mode with low radiation loss. The inter-antenna coupling within the array effectively suppresses electromagnetic leakage in the plane, allowing for a high-quality factor and low threshold current density even with only a few antennas in the array. As a result, the laser's power consumption is reduced to 950 mW. Moreover, the discrete antenna array design provides a larger heat dissipation area compared to the heat-generating area, and with the lateral heat dissipation channels offered by the unpumped regions, the thermal resistance per unit area is as low as 5.6 mK/W/cm². By significantly reducing power consumption and enhancing heat dissipation efficiency, the laser achieves a lasing frequency of 3.18 THz, a side-mode suppression ratio (SMSR) of 19.5 dB, and a beam divergence angle of 35°×35°. It operates continuously at 3.14 mW at 20 K, with a maximum continuous operation temperature of 90 K, notably higher than that of Fabry-Pérot cavity lasers made from the same material. This work provides a novel approach to improve the continuous operating temperature of THz-QCLs.