A plasmonic refractive index (RI) sensor based on photonic crystal fiber is proposed. A chemically stable thin film of gold (Au) is used as an active plasmonic layer and high RI material Ta₂O₅ is used as an overlayer over the gold thin film. The effect of Ta₂O₅ thin film on the sensor performance is analyzed in detail and a novel, as per the authors’ best knowledge, operating analyte RI range tunable property is reported. It is observed that the operating range is tuned toward the lower RI region with increasing Ta₂O₅ layer thickness. Furthermore, the sensor is optimized and its sensitivity is realized using both wavelength and amplitude interrogation techniques. A maximum wavelength sensitivity and amplitude sensitivity of 16,354 nm/RIU (refractive index unit) and 1574  RIU^−  1, respectively, are obtained corresponding to the operating RI range of 1.39 to 1.41 for the optimized structure. Moreover, the detection accuracy of the sensor is found of the order of 10^−  6 with a high figure of merit up to 282 representing an overall good sensing performance. The sensor performance is realized using surface plasmon resonance phenomenon and numerically analyzed using finite element method. Our study, no doubt, provides a new direction of designing RI sensor that could tune the RI range to the desired operating range. In addition, the simple design feasibility, low fabrication cost, and portable nature of the proposed sensor make it suitable for industrial and chemical sensing applications.