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The steep eastern margin of the Tibetan Plateau is thought to have developed in the Cenozoic by brittle crustal thickening or lower crustal flow related to India‐Asia collision. However, our data indicate that Late Triassic crustal shortening and voluminous magmatism contributed to crustal thickening and topography of the region. Sr/Y ratios of intermediate to silicic magmatic rocks reflect mineral assemblages in the magma source and can be used to constrain crustal thickness. The Sr/Y ratios from the Songpan‐Ganzi Fold Belt in the eastern Tibetan Plateau indicate that the eastern margin of the plateau achieved a crustal thickness ranging from 44 ± 6 to 67 ± 9 km during ca. 220 to 190 Ma. An average elevation of 2,600 ± 300 m at ca. 211 Ma is deduced based on average crustal thickness of 55 ± 2 km assuming Airy isostatic compensation. The thickened crust and high topography of the eastern margin of the Tibetan Plateau during the Late Triassic is supported by intense shortening and voluminous magmatism, contemporaneous Barrovian metamorphism, and the development of a foreland basin. Our data indicate that the eastern margin of the Tibetan Plateau was initially built during the Late Triassic, and this area has a long history of crustal thickening and mountain building. Key Points: Eastern margin of the Tibetan Plateau reached an average crustal thickness of 55 ± 2 km during ca. 220 to 190 MaThis margin attained an average elevation of 2,600 ± 300 m due to crustal thickening and magma underplating during this time intervalLate Triassic crustal thickening marks the initial growth of the present‐day eastern margin of the Tibetan Plateau [ABSTRACT FROM AUTHOR]