The binder is an indispensable battery component that maintains the integrity of the electrode. Polyvinylidene fluoride (PVDF) is most commonly used as a binder in rechargeable batteries; however, it is associated with the toxic and expensive N-methyl-2-pyrrolidone organic solvent. Here, through the cross-linking of sodium alginate (SA) with metal cations, a high-performance hydrogel binder is developed that maintains the stability of MnO2 cathodes in an aqueous electrolyte. Owing to the strong adhesion, high hydrophilicity, and good mechanical stability resulting from the strong bonding of Ca2+ with SA, a commercial microsized MnO2 cathode with a Ca−SA binder delivered a capacity above 300 mAh/g at 1 C, which was larger than those of Mn−SA and Zn−SA (∼200 mAh/g) and PVDF (∼150 mAh/g) binders, and a capacity of 250 mAh/g at 3 C for over 200 cycles. These encouraging results could unlock the enormous potential of aqueous binders for practical applications in aqueous batteries.