The Late Triassic was a key period for the evolution of the western Sichuan Basin from marine to continental sedimentation. However, the provenance of the earliest terrigenous sediments during this period remains debated, hindering our understanding of the tectonic events that ruled the evolution of the basin at that time. Herein, samples of fine sandstone from the Upper Triassic Ma'antang and Xiaotangzi formations in the western Sichuan Basin were collected for petrology, heavy mineral analysis, bulk rock geochemistry, and detrital U–Pb dating. In addition, corresponding data from potential source areas were collected for comparison. The sedimentological, geochronological, and geochemical characteristics of terrigenous sediments suggest that the clastic materials were mainly sourced from the Qinling orogenic belt and Yangtze Craton (including the northern and western margin). The Longmen Shan thrust belt likely provided clastics since the Early Norian. Siliciclastic deposits of the Late Triassic sedimentary succession of the western Sichuan Basin (Ma'antang and Xiaotangzi formations) yielded young zircon U–Pb ages of 214–245 Ma, suggesting that these zircons were likely sourced from the magmatic activities in the South Qinling orogenic belt or Yidun Island Arc. Combined with previous research, this study predates the transformation of the western Sichuan Basin from marine to continental sedimentation in the Late Carnian/Early Norian period. • The terrigenous clastics of the western Sichuan Basin first appeared in the Ma'antang during the Late Carian period. • The terrigenous clastics possibly derived from the Qinling Orogenic Belts, Yangtze Craton and its marginal blocks. • Since the deposition of the Xiaotangzi Fm., the Longmenshan thrust belt provided clastics for the Western Sichuan Basin. • The young volcanic zircons (214∼245Ma) were probably sourced from the South Qinling orogenic belt or Yidun Island Arc.