The arm-first approach is well accepted for the synthesis of core cross-linked star (CCS) polymers, which typically involves two separate steps, i.e., the initial synthesis and purification of the arm polymers followed by a second step synthesis of CCS. We report here a one-pot approach for the RAFT synthesis of a well-defined CCS in water without the separation and purification of the arm polymers. This novel approach consists of a homogeneous polymerization step to prepare the arm polymer, polyPEGMA, and a heterogeneous polymerization step to assemble the arm polymers into CCS polymers. Various parameters affecting the formation of the CCS were studied, including the type of cross-linker, spacing monomer, arm polymer concentration and arm polymer molecular weight. Cross-linkers of longer length provided the flexibility required to form high-quality CCS polymers. The use of spacing monomers in conjugation with cross-linkers reduced the stress during cross-linking and also provided enhanced driving force to produce well-defined CCS polymers. High-quality CCS polymers were prepared at relatively high polymer concentration (≥10%). The higher molecular weight arm polymer exhibited increased steric hindrance for CCS formation. Overall, this one-pot approach is highly efficient, environmentally benign and amenable to a variety of reaction conditions.