Interest is growing in the role that nanoparticles play in modifying the biological effects of contaminants. This study aimed to determine whether nano-TiO₂ exhibited pronounced influence on arsenate (As(V)) toxicity levels to the marine microalgae Nannochloropsis maritima. We compared individual and combined toxicity levels of As(V) and nano-TiO₂ by assessing the inhibition percentages of algal growth. Compared to groups treated with As(V) alone, an EC₅₀ of 53.0 mg/L decreased by 28.8% after the addition of nanoparticles. This enhanced toxicity was attributed to the inhibition of As methylation and the promotion of lipid peroxidation in the presence of nano-TiO₂. Additionally, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) also showed that algal cells exhibited different degrees of shrinkage, that cell wall were destroyed in the process, and that the photosynthetic apparatus was virtually indiscernible after the addition of nano-TiO₂. In addition, for low As(V) concentration exposure groups, nano-TiO₂ could alleviate As(V) toxicity to some extent by reducing As sorption onto algal cells and subcellular distribution in organelles, but this alleviation effect could not protect against the combined toxicity (both As(V) and nano-TiO₂) effect on N. maritima, which was verified by the higher inhibition percentage of the algal growth rate in the combined exposure group treatment compared to the As(V) exposure treatment alone. Our results suggest that more attention must be paid to the potential impact of nanoparticles on the bioavailability and biotransformation of contaminants in phytoplankton.