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Sediment provenance in the lower reach of rivers is important in reconstructing paleogeography and linking continental tectonic and climate changes with sedimentation in continental margins. This study presents 1300 new detrital zircon U-Pb ages from seven latest Eocene-Miocene sandstone and one modern river sand samples in south Texas and integrates these new data with published detrital zircon data to elucidate the evolution of the Rio Grande paleoriver during the Cenozoic. The new data contain a major population of the Cordillera magmatic province (22-280 Ma) and populations older than 280 Ma that were mostly recycled from sedimentary cover in the southern Rocky Mountains and southern Great Plains. The latest Eocene-Oligocene sandstones contain abundant air-fall zircons, and their detrital zircon maximum depositional ages constrain depositional ages. The changes in detrital zircon age distributions suggest that the paleoriver has changed its drainage extent three times, including a late Eocene drainage reduction by cutting off sediment supply from the west, a late Oligocene drainage reduction to the east of the southern Rocky Mountains and expansion to the East Mexico Arc in northeastern Mexico, and a latest Neogene drainage expansion to the west of the Rio Grande Rift and southern Colorado.