Phytoglycogen is a naturally occurring glucose polymer that is produced by sweet corn in the form of compact nanoparticles with a dendritic or tree-like architecture. The soft and porous nature of the nanoparticles, combined with their biodegradability and lack of toxicity, makes them ideal for a broad range of applications in personal care, nutrition, and biomedicine. To fully exploit these applications, it is necessary to understand the complex properties of the soft, hydrated nanoparticles in detail. In the present study, we have used atomic force microscopy (AFM) force spectroscopy to collect high-resolution force-distance maps of a large number of individual phytoglycogen nanoparticles, providing unique insights into the morphology and mechanical stiffness of the nanoparticles at the single-particle level. Our measurements performed in water on nanoparticles covalently bonded to gold surfaces revealed an inner branched structure and high deformability of the nanoparticles at modest values of the applied force. These measurements also allowed us to determine the spatial distribution of Young's modulus values within individual nanoparticles. Drying of the nanoparticles resulted in a dramatic increase in Young's modulus, quantifying the effect of hydration on their mechanical stiffness. We obtained excellent agreement between AFM and osmotic pressure measurements of the mechanical properties of hydrated phytoglycogen nanoparticles; the ratio of the average Young's modulus measured using AFM to the bulk modulus measured using osmotic pressure was in close agreement with that expected for a material with Poisson's ratio nu = 0. The soft, deformable nature of phytoglycogen nanoparticles revealed by our measurements provides new insights at the single-nanoparticle level and suggests their suitability for biomedical applications such as transdermal and targeted drug delivery.