Arsenic (As) is a toxic metalloid, widely distributed in soils, waters, atmosphere and organisms. Arsenic contamination in groundwater is a well-known global environmental problem over the past 10 years. The As in groundwater is released from soils following the development of anaerobic conditions, and arsenite [As(III)] is the predominant As species in the reducing environments. In many parts of the world, arsenic-contaminated groundwater is not just used for irrigation of crops but is also widely used for drinking water, particularly in arid regions where are often concomitant with high alkalinity and/or salinity, causing widespread public health concern. Microalgae are an important primary producer in aquatic systems with great potential in bioremediation of As due to their high As removal ability. Previous studies have demonstrated that Dunaliella salina, an exceptionally halotolerant microalga, is a good candidate in arsenate [As(V)] remediation due to its strong As(V) tolerance and As accumulation ability. However, the detailed As(III) tolerance and metabolism, especially the biooxidation of As(III) by D. salina, remain unexplored. Understanding the inherent mechanisms of As(III) oxidation in D. salina is important for maximizing the As remediation efficiency, since As(V) is much easier to remove from water than As(III) by conventional physicochemical methods such as adsorption, coagulation, coprecipitation, ion exchange, and membrane separation. Therefore, we investigated As(III) oxidation and toxicity in a marine microalga D. salina under different phosphate conditions. The results of 72-h As(III) toxicity experiment showed that D. salina is more sensitive to As(III) than to As(V), indicating that As(III) oxidation by this algae was acting as a detoxification process. The oxidation of As(III) appeared to be more effective with increased extracellular phosphate concentrations. We found that the higher As(V) percentage in the medium under the higher phosphate concentrations was mainly from the biooxidation by D. salina, which mainly occurred on the cell surface whereas intracellular oxidation cannot be neglected. Overall, the findings of this work not only demonstrate that phosphate play a key role in the oxidation of As(III) which occur both inside and outside the D. salina cells but also expand our knowledge of As detoxification mechanism in this marine alga.