Different stacking order in van der Waals materials has a large impact on the interlayer coupling and can lead to many novel physics. Recently, stacking-induced two-dimensional ferroelectricity has been observed in the zero-degree aligned hexagonal boron nitride (hBN) and graphene-hBN heterostructures. Here we report a spontaneous polarization induced photovoltaic effect in semiconducting MoS₂ in the 3R phase. We experimentally demonstrated that the spontaneous polarization is homogeneous throughout the exfoliated bilayer, which is much larger in size than the individual domain in artificially stacked homobilayers. A giant depolarization field induced by the spontaneous polarization allows us to build a scalable graphene-MoS₂ photovoltaic device. Utilizing the weak screening in graphene and the exciton-enhanced light-matter interaction and ultrafast interlayer relaxation in MoS₂, we demonstrate a high external quantum efficiency in the few-layer device. Our results indicate that transition metal dichalcogenides in the rhombohedral phase are a class of promising candidates for optoelectronic applications such as energy-efficient photo-detection with high speed and programmable polarity.