Biguanide-based cationic polyelectrolytes are used as key components of interpolyelectrolyte complexes bolstering alginate hydrogel microcapsules employed in cell therapies. Nevertheless, electrostatic complexation of these unique polycations has not been studied before. In this study, the interaction between biguanide condensates and anionic polyelectrolytes with carboxylate groups was studied on a model system of a metformin condensate (MFC) and an anionic diblock polyelectrolyte poly(methacrylic acid)-block-poly(ethylene oxide) (PMAA-PEO). The formation of MFC/PMAA-PEO core-corona nanoparticles was followed by static, dynamic and electrophoretic light scattering and by isothermal titration calorimetry and their internal structure was investigated by small angle neutron scattering and cryogenic transmission electron microscopy. It was found that the aggregation of PMAA-PEO chains induced by MFC occurred at much lower MFC/PMAA-PEO ratios that would correspond to the isoelectric point, thus yielding strongly negatively charged nanoparticles, suggesting the role of specific (non-electrostatic) interactions in the stabilization of the complex between PMAA and MFC.