Recently novel two-dimensional materials for solar water splitting have drawn enormous research attention because of their tunable electronic properties for enhancing photocatalytic performance. Here, based on extensive density functional theory calculations, we clearly reveal that single- and few-layer BiOI have favorable band gaps, light electron effective masses, excellent optical absorptions, and nice band edge alignments with the water redox potentials. Moreover, due to the weak interlayer interactions, these remarkable electronic properties are robust, and almost independent of the layer thickness of the BiOI nanosheets. Our theoretical results suggest that the BiOI nanosheets can be efficient photocatalysts for solar water splitting.