Carboxylate-functionalized imidazolium ionic liquids (CFIMILs) were synthesized via a one-pot quaternization reaction and serve as draw solutes for the first time for forward osmosis (FO) desalination. It provides a solution to simultaneously overcome the problems plaguing the use of pressure-driven membrane technologies to extract water from saline water - low water recovery efficiency and high reverse solute diffusion. The CFIMILs contain a substituted 5-membered imidazole ring and fully ionize into ionic species in water. These characteristics endow CFIMILs with capability of high water recovery efficiency and minimal reverse diffusion in FO. Sodium salt of 1-acetate-2,3-dimethyl imidazole ionic liquid (IL-1) at 0.8 M produces water fluxes of 63.6 LMH (PRO mode) and 29.5 LMH (FO mode) against the DI water feed, greatly outperforming conventional NaCl draw solute and other synthetic draw solutes. Efficient water recovery is achieved using IL-1 for seawater desalination with a water flux 5 times higher than that of NaCl. Particularly, CFIMILs draw solutes have negligible reverse diffusion in FO and are recycled completely via an MD process with good reproducibility when reused to FO, demonstrating great potentials of CFIMILs as a new class of draw solutes for FO water treatment.