In this paper, the atomic-scale three-dimensional structure of Pt nanoparticles were determined using high energy X-ray diffraction techniques and reverse Monte Carlo simulations. It was found that dendrimer encapsulated Pt nanoparticles are more ordered than those stabilized by long-chain alkanes. Furthermore, Pt nanoparticles measured in ambient dry conditions exhibit higher local atomic order compared to Pt nanoparticles measured in aqueous solutions. The atomic-scale structural information presented here can offer a more comprehensive understanding of the structure–property relationship for Pt nanoparticles and can be further used in the design of nanoparticles with improved properties.