Photocatalytic water splitting has emerged as a promising avenue for the utilization of solar energy to produce hydrogen. To advance the development of photocatalytic hydrogen production, the optimization and design of novel photocatalysts is a crucial task. In this study, ZnFe2O4 nanoparticles with iron-rich surfaces were synthesized through alkali treatment and employed in photocatalytic water vapor decomposition. The alkali-treated ZnFe2O4 displayed superior photocatalytic activity, with the optimized sample exhibiting an impressive H2 generation rate of 61.9 μmol·g−1 after 5 h of irradiation, which is significantly higher than the untreated ZnFe2O4 sample (15.9 μmol·g−1). This enhanced performance is attributed to the improved separation of photogenerated carriers and the improved adsorption of water molecules on the iron-rich ZnFe2O4 surface. This research may provide a valuable guide for the design of highly active ternary oxide/sulfide semiconductor photocatalysts for water vapor splitting.