Hygroscopic salt-hydrogel composite sorbents have attracted increasing attention for atmospheric water harvesting (AWH) applications but suffer from the salting-out effect. To this end, this work, for the first time, discovers that the salting-in effect possessed by a zwitterionic hydrogel is able to facilitate water vapor sorption by the hygroscopic salt under otherwise the same conditions. For demonstration, zwitterionic hydrogel of poly-[2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (PDMAPS) was synthesized, and the hygroscopic salt of LiCl was embedded into PDMAPS to produce the salt-hydrogel composite. LiCl salt not only endows the sorbent with high water vapor sorption capacity but also facilitates the dissociation of self-association between cationic and anionic groups of PDMAPS. This salting-in effect was evaluated and confirmed experimentally and via density functional theory (DFT) calculation. The salting-in effect renders the zwitterionic hydrogel matrix with enhanced swelling capacity, leading to the sorbent's high AWH performance. With a photothermal component of CNT integrated into the sorbent, a fully solar energy-driven AWH process was demonstrated outdoors. This study provides important guidance to the design of hydrogel-based AWH sorbents.