The incision of the Sanmen Gorge marks the birth of the modern Yellow River, but its timing varies from the late Miocene-early Pliocene to the late Pleistocene (∼0.15 Ma), and the suggested forcing mechanisms vary from the uplift of the Tibetan Plateau to global climate change. Here, we report sedimentologic, geochronologic, and provenance data from a drill core near the Sanmen Gorge, the last gorge along the main course of the Yellow River. Our results indicate that typical river channel deposits, with detritus from the Ordos Block in the upstream regions, started to accumulate in the Sanmen Gorge at ∼1.25 Ma. When integrated with river terrace evidence from the upstream and downstream regions, the results provide robust evidence that the final integration of the modern Yellow River occurred at ∼1.25 Ma, consistent with the beginning of the Mid-Pleistocene transition (MPT). We propose that the accelerated lowering of eustatic sea level during the MPT may play as important a role as tectonism in driving the birth and evolution of the modern Yellow River.