Fast recombination of photogenerated charge carriers is a major problem in the photoelectrochemical and photocatalytic processes. In this work, we report significantly improved PEC performance of a nanocomposite consists of In2S3 nanoparticles dispersed on g-C3N4 nanosheets synthesized by a simple and facile wet chemical route. The results of high-resolution TEM study show that the obtained In2S3 nanoparticles of size 10-20 nm exist in cubic phase and are uniformly dispersed on the surface of g-C3N4 nanosheets. The In2S3/g-C3N4 nanocomposite with 25 weight percentage of In2S3 exhibits 8.5 times higher photocurrent density than the single-phase g-C3N4 under visible light illumination. The enhanced photocurrent density exhibited by the In2S3/g-C3N4 nanocomposite is attributed to the efficient separation of photogenerated charge carriers. The charge transfer mechanism in In2S3/g-C3N4 heterojunction was studied by a series of experiments, such as electrochemical impedance spectroscopy, photoelectrochemical measurement and photoluminescence emission spectroscopy. The intimate interface promotes the charge transfer and inhibits the recombination rate of photogenerated electron-hole pairs, which significantly improves the photoelectrochemical performance. A detailed charge transfer mechanism is discussed based on the Mott-Schottky plot study. This heterojunction material is found to be an efficient photocatalyst for the degradation of both cationic rhodamine B dye and anionic methyl orange dye as the lifetime of photogenerated charge carriers is higher in the composite than in single-phase In2S3 and g-C3N4. A strong correlation between the photoelectrochemical and the photocatalytic performances is observed in this composite.