Polymeric nanocapsules templated on liquid cores were obtained without stabilization by low molecular weight surfactants and applied as photochemical reactors. Toluene cores of the capsules were stabilized by novel graft amphiphilic polyelectrolyte poly(allylamine hydrochloride)-graft-poly(vinylnaphthalene) (PAH-graft-PVN) that was synthesized via nitroxide-mediated radical polymerization. Self-aggregation of the photoactive copolymer consisting of a solvophilic backbone and solvophobic side chains in water as a selective solvent leads to formation of intermolecular aggregates in which hydrophobic perylene fluorophore can be solubilized. However, at the water/toluene interface the macromolecules partially uncoil and polymeric chains are able to stabilize emulsion droplets by anchoring the hydrophobic arms in them. Formation of such capsules was followed using dynamic light scattering and electron microscopies. The obtained capsules showed long term stability. Importantly, energy transfer between the polymeric naphthalene moieties and incorporated perylene chromophores was realized much more efficiently in the core–shell capsules than in the copolymer aggregates. This can be explained in terms of the less packed but still confined environment of the nanocapsules. Such an assembly favors migration and transfer of excitation energy, which is otherwise dissipated in naphthalene excimers and aggregates that dominate in the copolymer aggregates formed in an aqueous environment. The obtained nanocapsules were shown to serve as both stable nanocontainers and efficient nanoreactors for photooxidation of perylene as an exemplary reaction. The reaction photosensitized by the polymeric naphthalene chromophores proceeded at least 3 times faster in the capsules than in the aggregates, disregarding the pH of the medium. Such properties make these novel capsules promising for numerous applications concerning photosensitized reactions that may be carried out in an aqueous medium and benefit from the confinement effect.