We present a correlation for the viscosity of pentafluoroethane (R125) based on a compilation and critical assessment of the available experimental data. The correlation covers a wide range of fluid states, including the supercritical region. It is applicable from the triple point at 172.52 to 500 K, with pressures varying up to 60 MPa. The formulation includes a zero-density contribution, initial density dependence based on the Rainwater-Friend theory, and a residual contribution for higher densities that combines virial terms with a free-volume term, both being temperature-dependent. The estimated uncertainty of the viscosity correlation (coverage factor of 2) is 3% along the liquid-phase saturation boundary, 3% in the compressed liquid phase at pressures to 60 MPa, and 0.8% in the vapor.