The terahertz region (1-10 THz) has potential applications in many areas, such as chemical sensing, medical imaging and free-space optical communications. With the demonstration of terahertz sources, it is quite necessary to develop the detection technology in terahertz. Here we propose an electrically tunable quantum dot infrared photodetector to detect the terahertz region. The proposed detector applies a lateral electrical confinement on the quantum wells and forms a quantum disk in the quantum well area. The two-dimensional quantum confinement of quantum disk combining the vertical confinement from the quantum barrier forms a quantum dot structure. Using the energy states and intersublevel energy spacing in the quantum dot, the detector can be used to detect the terahertz region. Changing the lateral electrical confinement, the intersublevel energy spacing can also be tuned and in hence different wavelengths can be detected. Our modeling and simulation results show the tunability of peak detection wavelength of the photodetector from ∼3.3 to ∼6.0 THz with a gate voltage applied on the detector from -2 to -5 V. The peak absorption coefficients of the detection are shown in the range of 103 cm -1. Compared with current quantum dot photodetectors produced by self-assembled growth method the detector proposed here is easier to be tuned and the effective sizes have a much higher uniformity, because of using electrical confinement.