The timing of formation of the low‐gradient, internally drained landscape of the Tibetan Plateau is fundamental to understanding the evolution of the plateau as a whole. Well‐dated sedimentary records of internal drainage of rivers into lakes are used to reveal the timing of this evolution. Here we redate the youngest continental sedimentary successions of central Tibet in the Lunpola Basin and propose a new age range of ca. 35 to 9 Ma, significantly younger than previously thought. We demonstrate long‐standing internal drainage in central Tibet since the late Eocene and stable sedimentary environments, source regions, and low topographic relief since at least the early Miocene. We suggest that sediment aggradation of internal drainage and reduction of hillslope gradients by erosion dominate the formation of low‐relief landscapes and that the late Cenozoic drainage basins in central Tibet developed in response to flow in the lower crust and/or mantle lithosphere. Plain Language Summary Internal drainage of rivers into lakes is a characteristic of the high plateaus of the world and, most notably, the Tibetan Plateau. Internal drainage generates local perched base levels for Tibetan rivers, enabling geomorphic isolation from the rapidly incising rivers of the Himalaya and surrounding regions. However, the question of when the low‐relief plateau topography was initiated has been largely ignored, and its formation mechanism is controversial. Here we report a detailed investigation in the Lunpola Basin of central Tibet and propose a new depositional age range of ca. 35–9 Ma. We demonstrate that the internal drainage kept eroding the mountain ranges and filling the surrounding lowlands since at least the late Eocene. By no later than the early Miocene, a gentle landscape formed in central Tibet. The late Cenozoic basins in central Tibet developed in response to deep crustal or mantle flow and associated upper crustal deformation. Key Points Robust age constraints of the youngest continental stratigraphic unit from the Lunpola Basin in central Tibet are reported Aggradation and erosion of internal drainage dominated the formation of low‐relief topography in central Tibet by the early Miocene times Late Cenozoic drainage basins in central Tibet developed in response to flow in the lower crust and/or mantle lithosphere