The increasingly popular, lead-free perovskite, Cs3Bi2I9 has a vulnerable Bi3+ state under reductive potentials, due to the high standard reduction potential of Bi3+/Biδ+ (0 < δ < 3). Contrary to this fundamental understanding, herein, ligand-coated Cs3Bi2I9 nanodiscs (NDs) demonstrate outstanding electrochemical stability with up to -1 V versus a saturated calomel electrode in aqueous 0.63 M (5% v/v) and 6.34 M (50% v/v) hydroiodic acid (HI), with a minor BiI3 fraction due to the unavoidable partial aqueous disintegration of the perovskite phase after 8 and 16 h, respectively. A dynamic equilibrium of saturated 0.005 M NDs maintains the common ion effect of I-, and remarkably stabilizes ∼93% Bi3+ in 0.63 M HI under a strong reductive potential. In comparison, the hexagonal phase of bulk Cs3Bi2I9 disintegrates considerably in the semi-aqueous media. Lowering the concentration of synthetic HI from the commonly used ∼50% v/v by elevating the pH from -0.8 to 0.2 helps in reducing the cost per unit of H2 production. Our Cs3Bi2I9 NDs with a hexagonal lattice have 4-6 (002) planes stacked along the c-axis. With 0.005 M photostable NDs, 22.5 μmol h-1 H2 is photochemically obtained within 8 h in a 6.34 M HI solution. Electrocatalytic H2 evolution occurs with a turnover frequency of 11.7 H2 per s at -533 mV and outstanding operational stability for more than 20 h.