Two-dimensional semiconductors with layer thicknesses of a few nanometers to tens of nanometers have attracted tremendous research interest due to their fascinating electrical, optical, and optoelectronic properties and technologically important applications. In this work, two dimensional ternary ZnIn2S4 nanosheets which self-assemble into microflowers have been synthesized through a feasible chemical vapour deposition process. High-resolution transmission electron microscope (HRTEM) analysis reveals that the ZnIn2S4 nanostructure exhibits an extremely high phase purity and a single crystal nature free of dislocations, stacking faults or twins. UV-visible diffuse reflection measurements and room-temperature photoluminescence characterizations reveal that the ZnIn2S4 nanosheets have a strong visible absorption in the range of the energy band gap of 2.6 eV and a wide luminescence band covering the visible spectrum from the green to infrared regions. Photo-electrochemical tests using the ZnIn2S4 nanosheets as a photoanode suggest that ZnIn2S4 nanosheets have a sensitive photoresponse to irradiation with visible light and can thus be used for clean energy harvesting.