Evidence of late Pleistocene closed-basin conditions in equatorial African lakes has far-reaching implications for the history of the Nile River. There has been widespread acceptance that overflow of Lakes Victoria and Albert into the Nile occurred some time during the terminal Pleistocene, but recent suggestions that Lake Victoria remained a closed basin until ca. 7.2 ka imply a much longer period of greatly reduced White Nile discharge. This hypothesis has major consequences for current ideas about the river's paleohydrology, early human exploitation of riverine resources, Nile delta sedimentation, and late Quaternary genesis of sapropels in the eastern Mediterranean. Here we resolve the timing of Lake Victoria's overflow by using strontium isotopes as a tracer for water from the principal lakes and rivers of the Upper Nile. Our data demonstrate that overflow of Lakes Victoria and Albert into the Nile drainage network occurred no later than 11.5 14C ka.