Van der Waals epitaxy (vdWE) is an approach to grow crystal materials of high quality on the substrates even under a poor lattice matching condition. Graphene has been investigated to be a desirable buffer layer between substrates and layered materials to achieve vdWE. In this work, BiI3, a layered semiconducting material with high light-harvesting capability in the visible range, is successfully grown on graphene substrates via vdWE and presents highly orientated and ordered film. This is attributed to weak van der Waals interactions at the graphene/BiI3 heterojunction, which is verified by photoemission spectroscopy. In addition, the highly sensitive photodetectors with graphene/BiI3 hybrid channels perform a negative photocurrent response and an ultrahigh photoresponsivity of ~106 A W−1 under dim light, which is comparable to most previously reported photodetectors with graphene/semiconductor vertical heterostructures. Furthermore, due to nearly free interfacial traps and dangling bonds at graphene/BiI3 van der Waals heterojunctions, the graphene−BiI3 hybrid photodetectors exhibit significantly faster photocurrent rise time (<10 ms) and fall time (~500 ms) as compared with graphene-based hybrid photodetectors with CH3NH3PbI3 perovskite absorber layer (rise (fall) time of ~2 s (~1s)).