This work presents a fully printed paper-based humidity sensor fabricated by an office inkjet printer. The original black ink serves as the electrode material, while a saturated sodium chloride solution as the functional ink for preparing humidity sensors. The sensor performances are regulated by adjusting the printed cycles of the sensing layers. Furthermore, the potential applications of this paper-based humidity sensor in real-time respiratory monitoring and continuous skin moisture monitoring are demonstrated. This low-cost (each humidity sensor coats about 0.125 cents), environmentally friendly, and facile fabrication method provides a viable solution for the simplified and quantity production of disposable electronics.