Arsenic (As) is a highly toxic environmental pollutant, it is ranking first in the ‘Superfund List of Hazardous Substances’. However, human activities such as mining, the burning of fossil fuels, and pesticide application, have amplified the input of this metalloid in various coastal ecosystems around the world. Coastal environments in close proximity to densely populated, industrialized regions are expected to represent the ultimate sink for TiO₂ nanoparticles (TiO₂ NPs) and arsenate, which may be critical in driving coexisting pollutants transport and likely influence bioavailability of TiO₂ NPs-As. To date, there is only limited information available on the uptake, bioavailability and biotransformation of As(V) associated with TiO₂ NPs in organisms. To better assess the ecological impact of As(V) with the presence of TiO₂ NPs in coastal environments, the effect of TiO₂ NPs on arsenate uptake，bioavailability and biotransformation in mussels Perna viridis were investigated. Compared with previous study about traditional contaminants associated with nanoparticles, the present study provided a new exposure pattern to simulate the pollution status in the actual environment: Test organisms were exposured in different arsenate levels until reaching a steady state, subsequently, TiO₂ NPs were added while maintaining the original arsenate level. The results showed that the presence of nTiO₂ significantly promoted the accumulation of As in mussels Perna viridis under TiO₂ NPs-As exposure. The contents of As speciation demonstrated the As biotransformation in the mussels included As(V) reduction, methylation to monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA), and subsequent conversion to arsenobetaine (AB). The reduced organic species (AB, MMA and DMA) and arsenic methyltransferases (GST) activities of Perna viridis with the existence of TiO₂ NPs suggested the detoxification and methylated systems were inhibited. Moreover, organic osmolytes (glycine betaine and dimethylglycine) contents and enzymes activities were reduced after the addition of TiO₂ NPs, resulting in a decrease of organic osmolyte - AB proportions. The results showed that TiO₂ NPs also induced the disturbance in osmoregulation of Perna viridis. In summary, this study provides a new exposure pattern to elucidated the effects of TiO₂ NPs on arsenate uptake, bioavailability and metabolism in mussels Perna viridis.