Arsenic is a toxic element and should be removed from environmental water. In this study, Y(III)-incorporated magnetite (YMAGx) was synthesized by co-precipitation at different molar ratios of Y(III) to Fe(II) (x=Y/Fe=0.5, 1, or 2), and the adsorption of As(III) and As(V) was evaluated. The point of zero charge (PZC) and specific surface area of YMAGx increased with increasing x. YMAG2 adsorbed more than 97% of As(III) and more than 90% of As(V) in a wider pH range (pH 6.0-10.5) than YMAG0.5 and YMAG1. The adsorption isotherms of As(III) and As(V) on YMAGx fit well to the Langmuir adsorption model, and the positive correlation was obtained between the maximum adsorption capacities of arsenic and x. The maximum adsorption capacities of As(III) and As(V) using YMAG 2 were 1.12 mmol/g and 3.26 mmol/g, which were higher than those of adsorbents based on iron and yttrium previously reported. It was suggested that As(III) and As(V) were adsorbed on YMAG2 by the formation of inner-sphere complexes. As(III) adsorbed on YMAG2 was partially oxidized to As(V), while As(V) was adsorbed without a change in the valence.