Abstract: Arsenic is a toxic element and widely distributed in the natural environment. It can be transferred through the food chain and cause serious harm to ecosystem and human health. Arsenic pollution has become one of the most pressing environmental concerns in estuaries and nearshore areas. Marine organisms generally contain high concentrations of arsenic and are mainly composed of arsenobetaine and arsenosugars, showing unique mechanisms for bioaccumulation and transformation. Marine organisms can convert toxic inorganic arsenic into less harmful methylarsenate through the bio-methylation process, ultimately generating non-toxic organic arsenic. Compared to inorganic arsenic, organic arsenic is more bioavailable, which can lead to higher concentrations of arsenic in marine organisms. Based on this critical issue, we summarized the arsenic bioaccumulation and transformation patterns of phytoplankton, zooplankton, polychaetes, shellfish, shrimp, crabs, as well as marine fish. Furthermore, we analyzed the endogenous and exogenous factors influencing arsenic accumulation and transformation in marine organisms. Due to the complexity and specificity of high arsenic enrichment in marine organisms, we should pay more attention to the bioaccumulation, transformation, and molecular mechanisms of arsenic in different marine organisms in future research.