Halide-free, water-in-salt electrolytes (WiSEs) composed of potassium acetate (KAc) and zinc acetate (ZnAc$_2$) are investigated as electrolytes in zinc-ion hybrid supercapacitors (ZHSs). Molecular dynamics simulations demonstrate that water molecules are mostly non-interacting with each other in the highly concentrated WiSEs, while “bulk-like water” regions are present in the dilute electrolyte. Among the various concentrated electrolytes investigated, the 30 m KAc and 1 m ZnAc$_2$ electrolyte (30K1Zn) grants the best performance in terms of reversibility and stability of Zn plating/stripping while the less concentrated electrolyte cannot suppress corrosion of Zn and hydrogen evolution. The ZHSs utilizing 30K1Zn, in combination with a commercial activated carbon (AC) positive electrode and Zn as the negative electrode, deliver a capacity of 65 mAh g$^{−1}$ (based on the AC weight) at a current density of 5 A g$^{−1}$. They also offer an excellent capacity retention over 10 000 cycles and an impressive coulombic efficiency (≈100%).