Gd-based hybrid polyionic complexes are employed as versatile precursors to generate Prussian blue–like nanoparticles using two complementary strategies: (i) direct formation through the addition of K₄[Fe(CN)₆], and (ii) post-modification of preformed Prussian blue nanoparticles. The resulting colloids are characterized in terms of size, morphology, and colloidal stability. Their magnetic resonance relaxivity is then systematically evaluated to assess their performance as MRI contrast agents. These Gd-containing Prussian blue–like nanoparticles exhibit enhanced relaxivity compared to conventional molecular Gd complexes, highlighting their strong potential as efficient and safer MRI contrast agents.