The invention relates to a refractometer and to a method of measuring the refractive index of a medium. An incident light ray is directed onto an interface consisting of a medium of known refractive index and of the medium being studied. Then the intensity of the reflected light ray is measured. The incident and reflected light rays are propagated in an optical fiber section. The ratio between the intensity of the incident ray and the intensity of the reflected ray allows the refractive index of the medium studied to be calculated by means of Fresnel's formulas.