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Chinese marginal seas play a crucial role in sea-land interactions, but there is limited large-scale research on the occurrence and ecological risks of As in these areas. This study employed an extensive investigation on the spatial distribution and fractions of As in surface sediments across all four Chinese marginal seas, encompassing 111 sampling sites. The results revealed that the surface sediments exhibited elevated levels of total As content, surpassing those observed in other marginal seas worldwide. The HCl extractable fraction was found to be dominant in the Bohai Sea, Yellow Sea, and East China Sea, while the residual fraction prevailed in the South China Sea. Both environmental factors, including dissolved oxygen (DO), temperature, salinity, and manganese and iron contents in sediments, as well as biological processes such as nitrate-dependent As(III) oxidation, are likely significant contributors to variations in sedimentary As fractions. The distribution of As between reactive iron phases and reactive sulfur phases exhibited spatial variations across nearshore regions and offshore areas, indicating a shift in the relative roles of iron and sulfur in As retention. Generally, this study demonstrated that the ecological risks associated with As in surface sediments in the Chinese marginal seas ranged from low to moderate levels. The occurrence and distribution of As in these sediments were influenced by a complex interplay of environmental conditions, the geochemical cycling of iron and sulfur, and microorganism-mediated electron transfer processes., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2025 Elsevier B.V. All rights reserved.)