Van der Waals (vdW) heterostructures open up excellent prospects in electronic and optoelectronic applications. In this work, mixed-dimensional metal-halide perovskite/graphene heterostructures are prepared through selective growth of CH3NH3PbBr3 platelets on patterned single-layer graphene using chemical vapor deposition. Preferred growth of single-crystal CH3NH3PbBr3 platelets on graphene surfaces is achieved, which is accompanied by significant photoluminescence quenching. Raman spectra reveal that perovskite platelets cause p-type doping in the graphene layer. A significant Fermi level decrease of 272 meV in graphene is estimated, which corresponds to a high doping density of 7.5 × 1012 cm−2. Surface potentials measured by Kelvin probe force microscopy indicate a negatively charged perovskite surface under illumination, which is consistent with the upward band bending deduced from conducting atomic force microscopy measurements. Moreover, a field-effect phototransistor is fabricated using the perovskite/graphene heterostructure channel, and the increased Dirac voltage under illumination confirms an enhanced p-type character in graphene. These findings enrich the understanding of strong interface coupling in such mixed-dimensional vdW heterostructures and pave the way toward novel perovskite-based optoelectronic devices.