The thermally induced change of the birefringence of a high‐birefringence fused‐silica optical fiber, when exposed to a proton beam (2 MeV, 0.5–44 nA, 5‐mm spot diameter), was measured with a fiber‐optic polarimetric sensor. A short length (5 cm) of 125‐&mgr;‐diam, single‐mode, polarization‐maintaining optical fiber was put orthogonally to the beaminvacuoand the linear polarization of a HeNe laser beam launched into the fiber was analyzed by a polarizer and recorded by a photodetector. The response of the sensor both to a continuous and to a modulated proton beam was studied. The photodetector output, which varies with the cosine of the induced relative phase retardance between the fiber eigenmodes, provides a dc or an ac (10 Hz test modulation frequency) measurement of the beam current. This device acts as a proton beam intensity monitor, with minimum beam perturbation and a sensitivity of 0.1 nA. Its possible use also as a beam position monitor or as a wire‐scanner beam profiler is suggested.