Incorporation of metal nanoparticles (NPs) that can trigger localized surface plasmon resonance (LSPR) is an effective method for improving the performance of photovoltaics. Herein, Au NPdecorated graphene oxides (AuNP@GO) are synthesized and incorporated into halide perovskite photovoltaics (HPPVs) to improve their efficiencies. The resulting HPPVs exhibit enhanced power conversion efficiencies (PCEs) under illumination at 1 sun and the device enhancement can be attributed to the LSPR effects of the AuNP@GO nanocomposites. More interestingly, from the external quantum efficiency (EQE) spectra, we find two spectral wavelength regimes where the EQE values are improved. Because the Au NPs are positioned at the interface between the buffer layer and the perovskite active layer, we attribute such unique "interfacial" plasmonic effects to the fact that the Au NPs are affected by the two dielectric environments surrounding them. Because solar radiation covers a broad wavelength range, the interfacial plasmonic effects should extend the spectral range of enhanced light absorption. Further, the potential of the AuNP@GO nanocomposites to enhance the performance of HPPVs under indoor lighting conditions is also evaluated.