Titanium dioxide nanoparticles (nano-TiO₂) are widely used in consumer products. Nano-TiO₂ dispersion could, however, interact with metals and modify their behavior and bioavailability in aquatic environments. In this study, we characterized and examined arsenate (As(V)) accumulation, distribution, and toxicity in Daphnia magna in the presence of nano-TiO₂. Nano-TiO₂ acts as a positive carrier, significantly facilitating D. magna’s ability to uptake As(V). As nano-TiO₂ concentrations increased from 2 to 20 mg-Ti/L, total As increased by a factor of 2.3 to 9.8 compared to the uptake from the dissolved phase. This is also supported by significant correlations between arsenic (As) and titanium (Ti) signal intensities at concentrations of 2.0 mg-Ti/L nano-TiO₂ (R=0.676, P<0.01) and 20.0 mg-Ti/L nano-TiO₂ (R=0.776, P<0.01), as determined by LA-ICP-MS. Even though As accumulation increased with increasing nano-TiO₂ concentrations in D. magna, As(V) toxicity associated with nano-TiO₂ exhibited a dual effect. Compared to the control, the increased As was mainly distributed in BDM (biologically detoxified metal), but Ti was mainly distributed in MSF (metal-sensitive fractions) with increasing nano-TiO₂ levels. Differences in subcellular distribution demonstrated that adsorbed As(V) carried by nano-TiO₂ could dissociate itself and be transported separately, which results in increased toxicity at higher nano-TiO₂ concentrations. Decreased As(V) toxicity associated with lower nano-TiO₂ concentrations results from unaffected As levels in MSFs (when compared to the control), where several As components continued to be adsorbed by nano-TiO₂. Therefore, more attention should be paid to the potential influence of nano-TiO₂ on bioavailability and toxicity of co-contaminants.