This paper presents the design of a power detector including a readout circuit as well as a power coupler for built-in self-test and automatic level control applications at D-Band. In order to achieve high bandwidth operation, the frequency dependency of the coupler’s coupling factor is exploited to compensate the active device frequency limitations. The detector achieves 1dB resolution and flat responsivity characteristics $(\pm 10.1$% error) up to 15dBm input power within the entire D-band (110 - 170GHz). Maintaining a sensitivity of more than 1mV/dB for input power values as small as -10dBm at 140GHz, the circuit presents a dynamic range as high as 25dB. The power detector occupies an area of $300\mathrm{x} 380 \mu \mathrm{m}^{2}$, excluding pads, for a total DC power consumption of 4mW. To the best knowledge of the authors, the presented circuit shows the lowest error in responsivity in combination with highest dynamic range at D-Band among the state-of-the-art power detectors.