The evolution of fluvial drainage systems reflects interactions of tectonic uplift and climate change. Many previous studies have emphasized how Cenozoic tectonic uplift of the Tibetan Plateau has driven the reorganization of large-scale rivers. However, Cretaceous fluvial drainage configurations of the region are poorly constrained. In this study, we present a comprehensive provenance dataset of Lower Cretaceous deposits in the Lanping and Simao basins in southeast Tibet based on petrology, detrital zircon U-Pb dating and Lu-Hf isotopes. Our results show that the quartz-rich Cretaceous fluvial deposits yield major detrital age modes in the 2400-2600 Ma, 1700-1900 Ma, 750-1000 Ma, 400-490 Ma, and 200-290 Ma periods, and are mainly recycled from older siliciclastic strata exposed in the Southern Qiangtang terrane, Songpan-Garze terrane, Sichuan Basin, and Kangdian Basement. Zircon age clusters of 150-200 Ma and 200-290 Ma yield mostly negative and slightly positive εHf(t) values indicating volcanic detritus supplied from the Yidun Arc and Southern Qiangtang magmatic rocks. Lower Cretaceous fluvial units in the Sichuan, Lanping, Simao, Vientiane, Khorat, and Phuquoc basins share similar detrital zircon age spectra, which we interpret as the existence of a unified fluvial drainage system. We thus propose a transcontinental paleo-river with its headwater from the Southern Qiangtang and Songpan-Garze terranes, and northern Sichuan Basin and flow from Lanping and Simao basins to the Khorat Plateau. We suggest that a warm and humid climate, rather than tectonic uplift, drove rapid erosion rates of the source terranes and establishment of the transcontinental river.