The fabrication of the semiconductor photoanode with good chemical stability, sufficient light harvesting ability, and fast charge carrier separation and transportation capability is crucial for efficient photoelectrochemical (PEC) water splitting. Herein, we designed the nanocomposite based photoelectrode which is integrated by hydrothermally grown 3D hierarchical nanoflowers (NFs) of bismuth sulfide (Bi2S3) and conducting polymer polyaniline (PANI). The as-fabricated PANI/Bi2S3 photoanode has demonstrated much enhanced photocurrent density (~6.95 mA/cm2 at 0.8 V vs. NHE), which is ~5 fold higher in comparison to Bi2S3 photoanode (~1.30 mA/cm2 at 0.8 V vs. NHE). Also, the PANI/Bi2S3 photoanode has exhibited profoundly increased charge separation and injection efficiency of ~78.31% and ~90.18%, respectively. Furthermore, ~3 fold increased incident photon-to-current conversion efficiency (IPCE) value is achieved by PANI/Bi2S3 as compared to the Bi2S3 photoanode. The rational design of hierarchical semiconductor heterojunction, improved visible light absorption, efficient separation of photogenerated charge carriers, and faster charge transport properties altogether account for the enhanced PEC performance of the fabricated PANI/Bi2S3 photoanode. A possible schematic model for the significantly enhanced activity of the PANI/Bi2S3 photoanode for solar light induced PEC water splitting has also been explicated. These findings would provide new opportunities for understanding, exploration, and advancement of different other novel 3D hierarchical nanostructures based systems in the area of energy conversion, harvesting, and storage in the future.