Cation-condensed microgel-core star polymers with poly(ethylene glycol) (PEG)-based arms were designed as unimolecular polycationic nanocapsules (hosts) to encapsulate and stimuli-responsively release hydrophilic and anionic dyes (guests) in water. Typically, a cation-condensed star polymer (core cations: similar to 670/star) was directly synthesized in high yield (>90%) by the linking reaction of a PEG macroinitiator (1) with a quaternary ammonium cation-carrying linking agent (2) in ruthenium-catalyzed living radical polymerization. Analyzed by UV vis spectroscopy, the star polymer efficiently encapsulated various hydrophilic dyes carrying sulfonate anions (methyl orange: MO; orange G: OG; methyl blue: MB) in water (UV-vis: similar to 400 OG per a single star). The efficient dye encapsulation is due to the high concentration of quaternary ammonium cations in the core. The number of core-bound dyes increased with increasing the number of core-bound cations. The ligation structure of dyes within the core was proposed: the immobilization of one OG molecule involves two in-core ammonium cations. Additionally, stimuli-responsive release of dyes from cation-condensed star polymers was successfully achieved via ion exchange with NaCl aqueous solution.