A simple model for viscosity based on residual entropy scaling is proposed in this study. The thermodynamic model applied to calculate residual entropy is the Groupe Europeen de Recherches Gazieres (GERG) 2008 equation of state. Dimensionless pure-component viscosity as a function of residual entropy is correlated by a third-order polynomial. The component specific coefficients of the polynomial are adjusted to pure-component viscosity data. Averaged over all pure-component data points, deviation accounts to only 3.21%. Viscosity for mixtures is predicted by using the mole fraction weighted average of the pure-component coefficients in the polynomial. Excellent results are obtained for natural gas mixtures. The averaged deviation for all 2021 evaluated natural gas data points is as low as 3.09%. Compared to alternative mixture viscosity models, this purely predictive approach is among the best performing models for most studied natural gas mixtures and sour gas samples. The proposed approach is not limited to natural gas viscosity, that is, methane-rich multi-component systems. Accurate predictions are also presented for binary and ternary mixtures over wide temperature and pressure ranges.