In this letter, we present a fiber-optic current sensor for the measurement of plasma current in thermonuclear fusion reactors. The sensor includes a low-birefringence optical fiber placed around a section of the vacuum vessel that hosts the fusion reaction. The interrogating unit consists of a polarizationsensitive photon-counting optical time domain reflectometer that enables to determine the light polarization rotation induced by the plasma current. The measurement principle is demonstrated by means of the Jones formalism. The sensor has been experimentally validated on the Tore Supra tokamak fusion reactor for a plasma current range going from 0.6 to 1.5 MA. A maximum error of 13.50% has been observed for the lowest current.