A theory is presented to account for the increase in surface tension of water in the presence of electrolyte. Unlike the original "grand-canonical" calculation of Onsager and Samaras, which relied on the Gibbs adsorption isotherm and led to a result which could only be expressed as an infinite series, our approach is ''canonical'' and produces an analytic formula for the excess surface tension. For small concentrations of electrolyte, our result reduces to the Onsager-Samaras limiting law.