We report on an extrinsic surface plasmon-enhancedrefractometer based on cladding mode resonance excitation in a photonic crystal fiber (PCF) equipped with a straight fiber Bragggrating (FBG). First, we show that the lattice pitch and the air hole diameter of the PCF microstructure define the spectral locationof the excited cladding mode resonances. Second, we demonstrate that if the PCF parameters are properly selected, those resonancesare sensitive to increases in steps of 1×10−4 refractive index units(RIU) of the refractive index value close to that of water. To thebest of our knowledge, this is the first time that the sensitivity of PCF cladding mode resonances to refractive index changesin water-based solutions is reported. We achieved experimental values of 40.3 nm/RIU in terms of wavelength sensitivity and −801dB/RIU in terms of amplitude sensitivity. The performance of our sensor is therefore comparable to that of tilted FBGs in step-indexfibers used for water refractometry, which indicates the potential ofour PCF sensor for biosensing. In addition, the sensor fabricationdoes not require any post-processing such as etching or polishing,which allows preserving the integrity of the fiber probe. Finally, thenarrowspectrum within which the PCF operates, allows envisagingmulti-target detection with a single fiber probe by using cascadedwavelength-multiplexed gratings.