Aqueous rechargeable Zn-ion batteries (ARZIBs) have been attracting huge attention recently, where V-based materials with host layer structures and fast channels enable the efficient diffusion of metal ions, leading to excellent properties of Zn²⁺ storage. Several ammonium vanadates have been explored as potential cathodes, and their performance in ARZIBs varies considerably. Herein, we choose H₂O-intercalated NH₄V₃O₈ (NH₄V₃O₈·0.5H₂O) nanobelts, which are synthesized by a low-temperature hydrothermal process, and reveal that the electrochemical performance of NH₄V₃O₈ is strongly enhanced by the H₂O molecules intercalated in the layer structure. Indeed, the NH₄V₃O₈·0.5H₂O nanobelts exhibit a super-high capacity of 423 mA h g⁻¹ at 0.1 A g⁻¹, together with long-term stability (50.1% retention after 1000 cycles) at 1 A g⁻¹. The Zn//NH₄V₃O₈·0.5H₂O battery thus assembled delivers a high energy density of 353 W h kg⁻¹ at a power density of 114 W kg⁻¹, comparing favorably with most of the state-of-the-art V-based cathode materials reported for ARZIBs. As a promising cathode candidate for aqueous batteries, the reversible (de)intercalation of Zn²⁺ in the H₂O-intercalated NH₄V₃O₈·0.5H₂O gives rise to the formation of Zn₃(OH)₂V₂O₇·2H₂O, which helps retain the desired long-term stability.