Current　forward　osmosis　(FO)　membranes　are　unsuitable　for　arsenic　removal　from　water　because　of　their　poor　arsenic　selectivity.　In　this　study,　we　designed　and　synthesized　a　series　of　novel　imidazolium-based　ionic　liquids　via　one-step　quaternization　reactions　and　grafted　these　novel　compounds　on　to　conventional　thin-film　composite　FO　membranes　for　treatment　of　arsenic-containing　water.　The　newly　developed　ionic　membranes　contained　a　functionalized　selective　polyamide　layer　grafted　with　either　carboxylic　acid/carboxylate　or　sulfonate　groups　that　drastically　enhanced　membrane　hydrophilicity　and　thus　FO　water　permeation.　Ionic　membranes　modified　with　sodium　1-ethanesulfonate-3-(3-aminopropyl)　imidazolium　bromide　(NH2-IM-(CH2)2-SO3Na)　outperformed　pristine　membranes　with　higher　water　recovery　efficiency.　Exceptional　performance　was　achieved　with　this　ionic　membrane　in　FO　arsenic　removal　with　a　water　flux　of　11.0　LMH　and　a　rejection　higher　than　99.5%　when　1000　ppm　arsenic　(HAsO42-)　as　the　feed　with　a　dilute　NaCl　solution　(0.5　M)　as　the　draw　solution　under　the　FO　mode.　Ionic　membranes　developed　in　this　work　facilitated　FO　for　the　treatment　of　arsenic-containing　water　while　demonstrating　its　superiority　over　incumbent　technologies　with　more　efficient　arsenic　removal.　©　2019　American　Chemical　Society.