The (Na0.25Nb0.75)xTi1-xO2 (NNTO, with x = 0, 0.01, 0.02, and 0.05) powders were prepared by solid-state reaction and used as humidity sensing materials. The humidity sensitive properties of the NNTO based capacitive-type sensors were systematically investigated. Our results revealed that the humidity sensitivity increases with the increasing of doping level. An unprecedented high humidity sensitivity of 13,705 pF/%RH with good repeatability and fast response/recovery time were achieved in the sample with x = 0.05. The point defects of oxygen vacancies, NbTi• ions, and the secondary phase of NaNbO3 were argued to be responsible for the exciting humidity sensitive properties. Interestingly, the humidity sensitivity can be easily tuned by a small dc bias of 0–4 V. The depression of the humidity-induced Maxwell-Wagner relaxation and the neutralization of the charged point defects due to bias-injected electrons were found to be the cause of the adjustability. This work paves the way for developing capacitive-type sensors with adjustable and excellent performances.