The preparation of spherical, nanosized interpolyelectrolyte complexes by the interaction of hyaluronic acid-block-poly(ethylene glycol) (HA-b-PEG) with poly-L-lysine (PLL) at a stoichiometric charge-to-charge ratio is described. The complexation was studied by dynamic light scattering and cryogenic transmission electron microscopy for HA-b-PEG with different lengths of the HA and PEG blocks. A minimal molecular weight of the HA block (ca. 9 kDa) is necessary for efficient complexation, while a minimal molecular weight of the PEG block (ca. 5 kDa) is needed to prevent macroscopic aggregation. The formed nanoassemblies have hydrodynamic radii ranging from 45 to 150 nm with very low dispersity indices (D.I. = 0.02–0.05). They appear as soft objects with a nanogel structure and pronouncedly swell on addition of NaCl. The increasing ionic strength leads to disruption of the complexes above ca. 120–160 mM of NaCl. Decreasing the ionic strength by dialysis reforms the nanogels, demonstrating the reversibility of nanogel formation. Crosslinking with carbodiimide leads to nanoassemblies with a very well defined structure and high stability against ionic strength.