Neogene redbeds passing upward into upward-coarsening conglomerate and debris-flow deposits at the foot of the Kunlun Mountains record the change in paleoslope related to uplift of the surface of the northern Tibetan Plateau. Detailed magnetostratigraphy of a 4.5 km section near Yecheng in the western Kunlun Mountains shows that the change from deposition on distal alluvial plains to proximal alluvial fans occurred during the Gilbert reversed chron (4.5–3.5 Ma). The change in depositional facies was accompanied by an increase in sedimentation rate from an average ∼0.15 mm/yr between the earliest Oligocene and the earliest Pliocene to 1.4 mm/yr in the Gauss normal chron (3.6–2.6 Ma). We interpret the change in depositional facies and increase in sedimentation rate as indicating that the main uplift of the northwestern Tibetan Plateau began ca. 4.5 Ma.