We report a sub-terahertz (THz) detector based on a 0.25-µm-gate-length AlGaN/GaN high-electron-mobility transistor (HEMT) on a Si substrate with nanoantennas. The fabricated device shows excellent performance with a maximum responsivity (R v) of 15 kV/W and a minimal noise equivalent power (NEP) of 0.58 pW/Hz0.5 for 0.14 THz radiation at room temperature. We consider these excellent results as due to the design of asymmetric nanoantennas. From simulation, we show that indeed such nanoantennas can effectively enhance the local electric field induced by sub-THz radiation and thereby improve the detection response. The excellent results indicate that GaN HEMTs with nanoantennas are future competitive detectors for sub-THz and THz imaging applications.