This paper proposes a terahertz (THz) detector in a 180-nm standard CMOS process. The detector consists of a square loop antenna and an NMOS transistor. The antenna has two feed ports. One is connected to the source of the transistor and the other is grounded to provide the source a dc ground. To improve the power transfer efficiency between the antenna and the transistor, impedance matching between them is needed. It is concluded that in order to increase the voltage responsivity of the detector, impedance matching should be achieved by changing the impedance of the antenna rather than by changing the impedance of the transistor. The parasitic capacitance and inductance of the gate power supply line will affect the antenna-transistor impedance matching. An open microstrip transmission line connected to the gate is designed to eliminate this influence. Measurement results show that the detector can detect THz radiation in the frequency range of 220 to 299 GHz. At 244 GHz, the detector achieves a best voltage responsivity of 2497 V/W and a noise equivalent power (NEP) of 357 pW/Hz(1/2).