We describe the synthesis and characterization of a high-efficiency visible light Cu$2$O photocatalyst. Uniform cubic, octahedral and rhombic dodecahedral Cu$2$O nanocrystals with a size of 300–600 nm were synthesized using a simple hydrothermal method. Photocatalytic experiments performed for different water samples (methyl orange solution, toluene solution and industrial wastewater) demonstrate that the rhombic dodecahedral Cu$2$O nanocrystals were highly active when driven by low-power white LEDs as a light source. In comparison with other reported photocatalysts, the Cu$2$O nanocrystals reported here show a much higher reaction rate and lower electrical energy per order. The reaction rate and photoefficiency were found to be highly correlated with the irradiated photon flux, and the Cu$2$O nanocrystals displayed high efficiency in the degradation of aromatic organics. The Cu$2$O photocatalyst in this work has the potential to be used for a low-cost and high-efficiency green technology for wastewater treatment.