The potential applications of forward osmosis (FO) for arsenic (As) removal from water requires an improved understanding of As rejection performance. This study investigated the effects of As concentration and pH in the feed solution, the salt concentration in the draw solution, and surface orientation of a FO membrane on As rejection. The results showed that the pH of the solution played an important role in As rejection. Salt concentration in the draw solution significantly influenced the water flux through the membrane. The concentration polarization with respect to surface orientation was a key factor for As retention; FO operation using the membrane with the active layer oriented toward the feed solution (AL-FS) was more effective than using the membrane with the active layer oriented toward the draw solution (AL-DS).