Humidity is a physical quantity that evaluates the concentration of water vapor in the air. The detection of humidity has received extensive attention in various fields such as industry, agriculture, meteorology, and national defense. Among the many reported humidity detection methods, humidity sensors stand out due to their advantages of portability, easy fabrication, low cost, high sensitivity, and fast response. It is worth noting that the humidity detection limit is becoming more and more stringent in some special industries, and the dynamic range of humidity measurement can be extended from low relative humidity to ppm or ppb or even ppt level. However, the development of high-performance low humidity sensors still faces great challenges due to the very small amount of water molecules that the sensor can adsorb and the limitation of some harsh environments. So far, the literature on low humidity sensors is scarce, and there is no review on its research progress. In this case, a comprehensive and in-depth understanding of the research progress of low humidity sensors is of great significance, and it may provide new insights to promote the development of low humidity sensors. Therefore, this paper reviewed the research progress of low humidity sensors for nearly three decades. Firstly, the necessity of low humidity detection, trace moisture generators and various low humidity measurement methods were introduced. Then, the research status of different types of low humidity sensors was highlighted, including capacitance, resistance, QCM, MCL, SAW, optical fiber, fluorescence and other types.