Nanogels of poly(acrylic acid) have been synthesized using a new method, based on intramolecular cross-linking of linear macromolecules in aqueous solution, induced by short (a few microseconds) pulses of fast electrons. Simultaneous generation of a number of carbon-centered radicals along the polymer chain results in predominantly intramolecular recombination, with only a minor contribution of intermolecular cross-link formation and chain scission. This new method allows the synthesis of nanogels in a pure system consisting only of polymer and water, in the absence of monomers, cross-linking agents, etc. Adjusting the polymer concentration and dose of radiation, one can control the molecular weight and dimensions of nanogels. The products have been characterized by static light scattering, viscometry, and atomic force microscopy, while the nonclassical kinetics of intramolecular recombination has been followed by time-resolved spectroscopy. Nanogels of poly(acrylic acid) are much more resistant to free-radical-induced degradation than the parent linear chains.