Highly sensitive terahertz (THz) sensors for a myriad of applications are rapidly evolving. A widespread sensor concept is based on the detection of minute resonance frequency shifts due to a targeted specimen in the sensors environment. Therefore, cutting-edge high resolution continuous wave (CW) THz spectrometers provide very powerful tools to investigate the sensors' performances. However, unpredictable yet non negligible frequency drifts common to state-of-the-art CW THz spectrometers limit the sensors' accuracy for ultra-high precision sensing and metrology. Here, we overcome this deficiency by introducing an ultra-high quality (Q) THz microresonator frequency reference. Measuring the sensor's frequency shift relative to a well-defined frequency reference eliminates the unwanted frequency drift, and fully exploits the capabilities of modern CW THz spectrometers as well as THz sensors. In a proof-of-concept experiment, we demonstrate the accurate and repeated detection of minute resonance frequency shifts of less than 5MHz at 0.6THz of a THz microresonator sensor.