Photoelectrochemical (PEC) photodetectors convert light into electrical signals via PEC processes, providing high sensitivity, positioning them as promising candidates for next-gen optoelectronic devices. Over the past few years, there has been a significant interest in the field of photodetection regarding 2D materials, owing to their advantageous properties. In this study, the PEC photodetection of hydrothermally synthesized MoS2–Se nanocomposites have been systematically tested. Structural analysis confirms the coexistence of MoS2 and Se phases in the synthesized nanocomposites with crystallite sizes in the nanoscale dimension. Optical studies indicate that MoS2–Se nanocomposites exhibit broadened absorption spectrum extending to UV region, unlike pure MoS2 which absorbs solely in the visible region. This enhances their sensitivity for wide-band photodetectors with a small bandgap (1.01–1.34 eV). Optoelectronic study revealed superior performance of MoS2–Se thin film PEC device with efficient photoresponse with a responsivity of 38.3 μA/W and fast response time of 0.12 s. The novelty of this research resides in the integration of these unique features in MoS2–Se nanocomposites for PEC photodetectors, explored for the first time. The innovative aspect of this study involves employing spin coating to deposit 2D MoS2–Se thin films, which enhances control over film thickness and uniformity, thereby resulting in improved responsivity and photocurrent.