A fiber-optic sensor based on surface-plasmon resonance for the determination of the refractive index is used for measuring the degree of salinity of water. The transducing element consists of a multilayer structure deposited on a side-polished monomode optical fiber. Measuring the attenuation of the power transmitted by the fiber shows that a linear relation with the refractive index of the outer medium of the structure is obtained. The system is characterized by use of a varying refractive index obtained with a mixture of water and ethylene glycol. Experimental results show that the sensor can be used as a salinity-degree measurement device with environmental applications.