Nanoparticles with various morphologies such as micelles, worm‐like/rod‐like and spherical vesicles are made using a polymerization‐induced self‐assembly (PISA) approach via a one‐pot RAFT dispersion polymerization. On polymerization/self‐assembly, we report a concurrent highly efficient loading of guest molecules within the nanoparticle structures. Different nanoparticle morphologies, such as spherical micelles, worm‐like, rod‐like, and spherical vesicles, are achieved by gradually increasing the number‐average degree of polymerization (DPn) of the PST block via increasing polymerization times (in a poor solvent) as determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements. In parallel, a guest molecule (Nile Red) is encapsulated during the polymerization without disturbing the morphology or the polymerization kinetics. image