We present a highly sensitive salinity sensor realized with a polyimide-coated polarization-maintaining photonic crystal fiber (PM-PCF) based on a Sagnac interferometer configuration. The achieved salinity sensitivity is as high as 0.616 nm/M which is 37 times more sensitive than that of previous reported polyimide-coated fiber grating sensor. It has a low temperature sensitivity of -0.0122 pm/°. The performance of the sensor in aqueous solution of NaCl with concentrations up to 5.12 mol/L has been experimentally investigated. The proposed fiber optic salinity sensor is a promising candidate for salinity measurement.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.