Highly porous magnesium oxide (MgO) nanoflakes were synthesized by the calcination of magnesium hydroxide nanoflakes created by a hydrothermal process. These MgO nanoflakes have a high specific surface area at 115.9 m2/g, and a total pore volume of ∼0.254 cm3/g. They demonstrated an exceptional As(III) removal performance from aqueous solutions, and their maximum sorption capacity could reach 506.6 mg/g, much higher than most reported values from other metal oxide nanomaterials. Such a high As(III) sorption capacity was found to depend on the in situ formation of Mg(OH)2 owing to the interaction of MgO nanoflakes with water. While preserving most of the large surface area of MgO nanoflakes, the in situ formed Mg(OH)2 had high affinity to As(III) in aqueous environment, and could react with As(III) to form a magnesium–arsenite compound, which is ultimately responsible for the exceptionally high As(III) sorption capacity of MgO nanoflakes.