Flexible and wearable electronic sensing devices play an increasingly important role in human health monitoring and non-contact sensors towards human to machine interface technologies. Humidity sensors have been proven to be effective in real-time respiratory monitoring and diagnostic analysis. The p–n junction of borophene–MoS₂ has been theoretically predicted to have an ultrahigh humidity sensing characteristic, but the experiment is still not reported. Herein, we demonstrated experimentally a borophene–MoS₂ heterostructured sensor with ultrahigh sensitivity, fast response, long life, good flexibility and high selectivity. Its sensitivity at a relative humidity (RH) of 97% is as high as 15 500%, which is more than 90 or 70 times higher than that of borophene or MoS₂ alone. The sensitivity is the highest among all the reported chemiresistive sensors based on 2D materials. The corresponding swift response and recovery times are much lower than those of borophene or MoS₂ alone. Additionally, the sensor shows superior flexibility under bending strain. Compared with other organic vapors, the sensor for water vapor exhibits outstanding selectivity. Besides, we also demonstrate the multifunctional applications of the sensor in human breath diagnosis and non-contact switch sensing systems, which indicates the potential of borophene-based sensors in future health care and human–machine interaction systems.