Humidity sensors are of significance in various fields, such as environmental and food quality monitoring, industrial processing, wearable and flexible electronics, and human health care. High-performance humidity sensors with high sensitivity, rapid response time, and good stability are of paramount importance in humidity sensing. In this paper, diversiform humidity sensors with different sensing mechanisms are summarized, including resistive, impedance, capacitive, quartz crystal microbalance (QCM), surface acoustic wave (SAW), field-effect transistor (FET), and optical fiber humidity sensors. Versatile nanomaterials such as graphene, transition-metal chalcogenide, MXenes, black phosphorus (BP), boron nitride (BN), polymers, and nanofibers were promising building-blocks for constructing humidity sensors. The latest progress in the wearable and flexible humidity sensors, and self-powered humidity sensors was summarized. The diversiform applications of the humidity sensors with great prospects were demonstrated in various fields in terms of human respiratory monitoring, skin dryness diagnosing, fingertip approaching, and non-contact switch. Moreover, the challenges and prospects of nanomaterials-based humidity sensors were discussed.