Platinum nanoparticles stabilized by an imidazolium-based cross-linked polymer (with chloride as the counteranion) efficiently catalyzed the oxidation of 5-hydroxymethylfurfural to form 2,5-furandicarboxylic acid in water under mild conditions with oxygen as the oxidant. This catalyst system is explored herein by varying the counteranion, that is, replacing chloride by BF4-, PF6-, bis(trifluoromethylsulfonyl)imide, hexanoate, or laurate anions, in the cationic polymer. The counteranion influences the structure of the obtained platinum nanoparticles, the surface electronic properties, and their catalytic activity. The highest reaction rates were obtained with the weakly nucleophilic bis(trifluoromethylsulfonyl)imide anion, which also favored platinum in the zero oxidation state, leading to complete conversion of the substrate and a high yield of 2,5-furandicarboxylic acid under mild conditions.