We have observed a far‐infrared (0.3–2.5 THz) radiation‐induced photovoltaic signal in an antenna‐coupled quantum point contract, which oscillates with the gate voltage and peaks at the onset of each subband. The polarity of this photovoltaic signal can be reversed by shifting the far‐infrared beam from the drain to the source, or vice versa. This signal has been unambiguously attributed to thermopower generated by asymmetric heating of the drain and the source by the far‐infrared radiation. Quantitative agreement has been obtained between measurements and calculations based on ballistic transport in one‐dimensional electron systems and the electrical and thermal circuit elements in our experimental system. ©1995 American Institute of Physics.