CoP was synthesized in situ on the surface of CdS nanowires using a cobalt-ammine complex as a precursor. The generated CoP@CdS photocatalyst was well characterized by X-ray diffraction, transmission electron microscopy, selected-area electron diffraction, and diffuse reflectance spectra. A more efficient charge transfer was successfully achieved owing to the close contact between CoP and CdS. The hybrid photocatalyst showed excellent activity for the reduction of nitroarenes to the corresponding anilines or azobenzene compounds in water. In present work, hydrogen evolution and nitro reduction were concurrent and with the consumption of substrate, the rate of hydrogen evolution increased. The driving force for the reduction originated from the activated hydrogen species in the photocatalytic reaction rather than from dihydrogen. The photocatalytic activity of as-prepared CoP@CdS in situ is comparable to that of the catalysts formed using a noble metal loaded onto CdS. Mechanistic investigation showed that the condensation route is the major pathway for nitro reduction in the present system, and azo compounds could be obtained with less irradiation time, while aniline will be obtained via long-time irradiation.