Metallic nanoparticles exhibit exceptional optoelectronic properties with applications in plasmonics, biosensing and nanomedicine(1-5). Recently, new synthesis techniques have enabled precise control over the sizes and shapes of metal nanoparticles(6-8), occasionally leading to morphologies that cannot be properly characterized using standard techniques. An example is five-fold-twinned decahedral Au nanoparticles, which are intrinsically strained as a result of their unique geometry. Various competing models have been proposed to predict the strain states of such nanoparticles. Here, we present a detailed analysis of the internal structure of a decahedral Au nanoparticle using aberration-corrected high-resolution electron microscopy and strain mapping. Our measurements confirm the presence of a disclination, which is consistent with the commonly accepted strain model. However, we also observed shear gradients, which are absent from the models. By comparing our local strain determinations with finite-element calculations, we show the effect of elastic anisotropy on the strain state in these nanoparticles.