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The Cathaysian Coastal Mountains are thought to be an ancient high‐topographic feature that existed along the margin of South China. They are characterised by extensive Jurassic–Cretaceous magmatism; however, their formation mechanism and timing remains uncertain. In this paper, we present sedimentological and detrital zircon analyses from Cretaceous–Eocene strata and drainage sediments from Hainan Island. Our analyses show a change in provenance from a proximal Cathaysian Coastal arc source in Cretaceous strata to a widely distributed intra‐island granite source in the Eocene strata and modern river sediments. Reconstruction of the crustal thickness evolution for the South China margin from Eu/Eu*‐in‐zircon proxy shows significant crustal thickening during the Late Jurassic–Cretaceous, thereby suggesting the existence of the Cathaysian Coastal Mountains. Together with a compilation of detrital zircon U‐Pb ages in the northern basins of the South China Sea margin, we conclude that the Cathaysian Coastal Mountains extended from the Red River region to Taiwan along the South China margin during the Late Jurassic–Cretaceous and then collapsed/eroded from northeast to southwest during the Eocene–Miocene. This large mountain range supplied detrital sediments to the marginal basins in the northern South China Sea and formed a topographic barrier that prohibited moist Pacific air to reach the relatively arid inland area of South China during the Late Jurassic–Cretaceous. [ABSTRACT FROM AUTHOR]