Between its headwaters in Colorado and the U.S-Mexico border, the Rio Grande is characterized by a 50-fold increase in total dissolved solids content.  Despite a century of research, the reasons for this salinization have remained obscure.  We propose that salinization may be explained by the mixing of deep-basin brine discharge with surface water.  To examine this hypothesis, we utilized salt-burden calculations and environmental tracers including d ¹⁸O, d²H, Cl^-, the Cl/Br ratio and the ³⁶ Cl/Cl ratio.  Evapotranspirative concentration of salts, indicated by enrichment of d¹⁸O and d²H, is insufficient to account for the total salinization.  An increase in the Cl/Br ratio from nearly 50 in the headwaters to greater than 1,000 at the southern end of the basin, in combination with a decrease in the ³⁶Cl/Cl  ratio with flow distance, implies significant saline subsurface water (commonly distinguished by high Cl/Br ratios and low ³⁶Cl/Cl ratios) contribution to the river.  Furthermore, increases in Cl^- and the Cl/Br ratio are focused at the southern ends of sedimentary basins of the Rio Grande rift, suggesting that deep saline groundwater enters the river where it is forced to the surface by bedrock highs.  A saline pool in San Acacia, New Mexico at the southern terminus of the Albuquerque basin exhibits the high Cl/Br ratio and low ³⁶Cl/Cl ratio characteristic of deep saline groundwater.  This confirms the important role of deep-basin brine discharge in the salinization of this major river system.