This article presents a highly integrated 300-GHz frequency-modulated continuous wave radar sensor using a custom-developed dual-function transceiver MMIC. The system can either be configured as a stand-alone ultra-wide-band radar sensor or as a flexible RF front-end, enabling up-conversion and down-conversion of modulated signals to and from the terahertz range. The transceiver MMIC is manufactured using a 90 nm SiGe BiCMOS process, featuring high-speed hetero-junction bipolar transistors with an $f_{\rm{T}}$ of 300 GHz and $f_{\rm{max}}$ of 520 GHz. Using on-chip antennas and a focusing lens, the EIRP of the system for radar operation is greater than 3.2 dBm in a bandwidth of 54 GHz. The full potential of the system’s 90 GHz tuning range is demonstrated in radar measurements. A calibration method is applied to expand the usable tuning range, achieving an extraordinary spatial resolution of 1.97 mm with a frequency sweep from 330 to 240 GHz in 5 ms for a target at a distance of 0.35 m. The potential industrial use of this spatial resolution is demonstrated in a plastic thickness measurement scenario. Additionally a 100 Mbd OKK communication link with a BER of 0.55% is presented using two systems at 0.3 m distance.