Metal halide perovskites have emerged as novel and promising photocatalysts for hydrogen generation. Currently, their stability in water is a vital and urgent research question. In this paper a novel approach to stabilize a bismuth halide perovskite [(CH₃)₂NH₂]₃[BiI₆] (DA₃BiI₆) in water using dimethylammonium iodide (DAI) without the assistance of acids or coatings is reported. The DA₃BiI₆ powder exhibits good stability in DAI solutions for at least two weeks. The concentration of DAI is found as a critical parameter, where the I⁻ ions play the key role in the stabilization. The stability of DA₃BiI₆ in water is realized via a surface dissolution-recrystallization process. Stabilized DA₃BiI₆ demonstrates constant photocatalytic properties for visible light-induced photo-oxidation of I⁻ ions and with PtCl₄ as a co-catalyst (Pt-DA₃BiI₆), photocatalytic H₂ evolution with a rate of 5.7 μmol·h⁻¹ from HI in DAI solution, obtaining an apparent quantum efficiency of 0.83% at 535 nm. This study provides new insights on the stabilization of metal halide perovskites for photocatalysis in aqueous solution.