Graphene p-n junctions grown by chemical vapor deposition hold great promise for the applications in high-speed, broadband photodetectors and energy conversion devices, where efficient photoelectric conversion can be realized by a hot-carrier-assisted photothermoelectric (PTE) effect and hot-carrier multiplication. However, the overall quantum efficiency is restricted by the low light absorption of single-layer graphene. Here, we present the first experimental demonstration of a plasmon-enhanced PTE conversion in chemical vapor deposited graphene p-n junctions. Surface plasmons of metallic nanostructures placed near the graphene p-n junctions were found to significantly enhance the optical field in the active layer and allow for a 4-fold increase in the photocurrent. Moreover, the utilization of localized plasmon enhancement facilitates the realization of efficient PTE conversion of graphene p-n junction devices under global illumination, which may offer an avenue for practical applications of graphene-based photodetectors and solar cells.