A substantial broadband increase in the external quantum efficiency (EQE) of thin-film organic photovoltaic (OPV) devices using near-field coupling to surface plasmons is reported, significantly enhancing absorption at surface plasmon resonance (SPR). The devices tested consist of an archetypal boron subpthalocyanine chloride/fullerene (SubPc/C60) donor/acceptor heterojunction embedded within a planar semitransparent metallic nanocavity. The absorption and EQE are modeled in detail and probed by attenuated total internal reflection spectroscopy with excellent agreement. At SPR, the EQE can be enhanced fourfold relative to normal incidence, due to simulated ninefold enhancement in active layer absorption efficiency. The response at SPR is thickness-independent, down to a few monolayers, suggesting the ability to excite monolayer-scale junctions with an EQE of ≈6% and a 16-fold absorption enhancement over normal incidence. These results potentially impact the future design of plasmonically enhanced thin-film photovoltaics and photodetectors and enable the direct analysis of the dynamics of photocurrent production at OPV heterojunctions.