A highly-matched semiconductor/cocatalyst is crucial to enhance the charge separation and surface reaction kinetics of the photoelectrodes in the solar water splitting system. In this paper, ultrathin NiMn LDH nanosheets were synthesized by a facile solvent exfoliation method and used to construct alpha-Fe2O3/ultrathin NiMn LDH heterostructures. When these heterostructures were used as photoanodes for photoelectrochemical water splitting, significantly improved photoactivity and stability were achieved. Compared to pristine alpha-Fe2O3, 15-fold enhanced applied bias photon-to-current efficiency (ABPE) was achieved for alpha-Fe2O3/ultrathin NiMn LDH heterostructures. According to the electrical impedance spectroscopic and Mott-Schottky measurements, the enhanced photoelectrochemical performance was ascribed to the presence of ultrathin NiMn LDH nanosheets as oxygen evolution cocatalysts and the strong interfacial interactions between alpha-Fe2O3 and ultrathin NiMn LDH nanosheets. It is expected that this strategy can be extended to other heterostructures for advanced performance in the fields of energy conversion and storage.