Developing efficient photocathodes with novel design is essential for enhancing the functioning of photoelectrodes in photoelectrochemical (PEC) water splitting. The efficiency of solar-to-fuel conversion has been proven to be improved by using a suitable structural composition to create heterostructures. Apart from surface reactions, charge transfer between heterojunction interfaces is critically important. We report the first-ever novel and rational design of a hybrid photocathode using a CuBi2O4 based absorber material with a Sb2S3 heterojunction and evenly dispersed plasmonic Au nanoparticles (NPs) sandwiched between CuBi2O4 and Sb2S3. The heterostructure comprising CuBi2O4/Sb2S3 revealed an enhanced photoactivity due to ameliorated light absorption and charge separation showing a photocurrent density of −2.25 mA cm−2 at 0 V vs. RHE at pH 6.65. The crucial dual role of sandwiched Au NPs, as an electron relay mediator, facilitates the electron transfer at the heterojunction interface. Secondly, a plasmonic sensitizer enhances light absorption and charge carrier concentration via charge injection in CuBi2O4/Au/Sb2S3. The CuBi2O4/Au/Sb2S3 photocathode displayed a remarkable photocurrent density of −3.2 mA cm−2 at 0 V vs. RHE (0.85% HC-STH at 0.45 V vs. RHE) at pH 6.65, two-fold enhancement compared to CuBi2O4 (−1.5 mA cm−2 at 0 V vs. RHE, 0.27% HC-STH at 0.3 V vs. RHE). The high-performance CuBi2O4/Au/Sb2S3 photocathode achieves the highest photocurrent and HC-STH efficiency for a heterojunction to the best of our knowledge. Our findings will pave the way for developing new photoelectrodes with metal NPs sandwiched between semiconductor heterostructures and increasing PEC performance for solar-driven PEC water splitting. © 2022 The Royal Society of Chemistry.