Two-dimensional transition-metal dichalcogenides (TMDCs) are notable materials owing to their flexibility, transparency, and appropriate bandgaps. Because of their unique advantages, TMDC p–n diodes have been studied for next-generation electronics and optoelectronics. However, their efficiency must be increased for commercialization. In this study, we demonstrated a heterostructure composed of few-layer ReS₂ and WSe₂. This few-layer ReS₂/WSe₂ heterostructure exhibits a p–n junction and an n–n junction in different gate-bias regimes. In the p–n junction regime, the heterostructure shows outstanding rectification behavior. Additionally, we identify three carrier-transfer mechanisms − direct tunneling, Fowler–Nordheim tunneling, and the space charge region − depending on the drain bias. Furthermore, the photovoltaic effect is observed in this few-layer ReS₂/WSe₂ heterostructure. As a result, a high fill factor (≈ 0.56), power conversion (≈ 1.5%), and external quantum efficiency (≈ 15.3%) were obtained. This study provides new guidelines for flexible optoelectronic devices.