© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Palladium nanoparticles stabilized by heteroatom donor-modified polystyrene-based polymer immobilized ionic liquids (PdNP@HAD-PIILP; HAD-PPh2, OMe, NH2, CN, pyrrolidone) are highly efficient catalysts for the Suzuki-Miyaura cross-coupling in aqueous media under mild conditions. Catalyst modified with phosphine was consistently the most efficient as it gave high yields across a range of substrates under mild conditions at low catalyst loadings. Incorporation of polyethylene glycol into the phosphine modified immobilised ionic liquid support improved catalyst efficacy by improving dispersibility and facilitating access to the active site. Moreover, each of the heteroatom modified catalysts was more active than the corresponding unsubstituted imidazolium-based polystyrene benchmark as well as commercial samples of Pd/C. Catalyst generated in situ from either [PdCl4]@PPh2-PIILP or its PEGylated counterpart [PdCl4]@PPh2-PEGPIILP, by reduction with phenylboronic acid, outperformed their pre-formed counterparts for the vast majority of substrates examined. The turnover frequency of 16,300 h−1 obtained at room temperature is one of the highest to be reported for palladium nanoparticle-catalysed Suzuki-Miyaura cross-coupling between 4-bromoacetophenone and phenylboronic acid in aqueous media under such mild conditions. (Figure presented.).