Boron‐rich particles with the boron fraction ca.10–20 wt % of controllable shape and size that can be easily prepared via simple ion co‐assembly are promising material for tumor treatment by boron neutron capture therapy. Electroneutral, dynamic core‐shell polymeric nanoparticles were prepared by co‐assembly of cationic PEO‐block‐PGEA diblock copolymer with sodium closo‐dodecaborate, Na2[B12H12]. This is the first example of polymer nanoparticles based on [B12H12]2−nano‐ion pairing. The high [B12H12]2− loading is proven by calorimetry at physiological salt concentration. As a result of rational design, rod‐, worm‐ and sphere‐like particles were produced and further tested using human glioblastoma and cervical carcinoma cell lines. Rod‐like particles yielded the highest internalization capability in all tested cell lines.