Defects in nanocrystals can dramatically alter their physical and chemical behavior. It is thus crucial to understand the defect behavior at the nanoscale to enhance material properties. Here, we report three-dimensional defect characterization at the onset of plasticity in a 550 nm Pt nanoparticle. By combining in situ nano-indentation with Bragg Coherent X-ray Diffraction Imaging (BCDI), we directly observe the strain field inside the Pt particle during indentation, revealing the nucleation and propagation of prismatic dislocation loops. Subsequent post mortem imaging of the complete dislocation network, coupled with multi-reflection BCDI, enabled us to determine the Burgers vectors of the defects revealing sessile dislocations. Finally, by measuring the elastic field inside the crystal during indentation, we estimate that the shear stress required to generate defects is 6.4 GPa, representing the upper theoretical limit of elasticity and setting an unprecedented standard for Pt nanoparticles. Our findings provide fundamental insights into defect dynamics in nanoscale systems, offering invaluable knowledge for advanced materials design and engineering.