The main objective of the present work is to develop an optical fiber relative humidity (RH) sensor having a linear response throughout over the widest possible dynamic range. We report an optical fiber RH sensor based on the evanescent wave absorption spectroscopy that fulfills this objective. The fiber sensor employs a specific nanoparticle (zinc oxide) doped sol–gel nanostructured sensing film of optimum thickness, synthesized over a short length of a centrally decladded straight and uniform optical fiber. A detailed experimental investigation is carried out to analyze the sensor response/characteristics. Fiber sensor response is observed to be linear throughout the dynamic range as wide as 4% to 96% RH. The observed linear sensitivity for the fiber sensor is $0.0012{\mathrm{RH}}^{-1}$ . The average response time of the reported sensor is observed to be as short as $0.06\mathrm{s}$ during the humidification. In addition, the sensor exhibited a very good degree of reversibility and extremely high reliability as well as repeatability.