Humidity monitoring plays a key role in the human-machine interface (HMI) and health-related devices. Current inorganic sensing materials provide a stable and fast method to monitor relative humidity (RH). However, these materials require complicated preparation processes and even toxic reagents. Organic humidity-sensing materials were considered to be biocompatible, but they face slow response time and cannot be used in real-time systems. There is still a great challenge to seek biocompatible humidity-sensing materials with high performance. Micro-nano structures formed by self-assembled peptides have been proven to have good optical, mechanical, semiconductive properties and intrinsic biocompatibility, which can be used for monitoring human health and activities. Herein, we reported an environmentally friendly humidity sensor based on peptide self-assembled micro-nano fibers. This humidity sensor was fabricated by a drop-coating method, in which the peptide fiber served as the humidity-sensing material and the Au electrode was used as a substrate. Self-assembled fiber networks exhibit excellent absorption of water molecules, which result in an extraordinary humidity sensitivity of more than 30,000 as well as ultrafast response (66 ms). To demonstrate the multifunctional possibilities, peptide humidity sensors were applied to respiration monitoring, non-contact switch, and baby diaper wetting monitoring. Our results provide useful strategies for detecting humidity changes in physiological activities based on peptide self-assembled fibers. These results indicated that peptide could be a promising candidate for human-machine interaction and healthcare humidity monitoring.