AbstractWe demonstrate here that the properties of poly (2‐(methacryloyloxy) ethyl trimethylammonium chloride) brushes can be tuned by counterion species. When the brushes are exposed to external chloride (Cl−) counterions, obvious dehydration and collapse are only observed at high salt concentrations. In the presence of very strongly chaotropic perchlorate (ClO4−), the brushes strongly dehydrate and collapse at a very low salt concentration. For the strongly chaotropic thiocyanate ion (SCN−), the changes in hydration and conformation of the brushes are similar to those observed for ClO4− but at a smaller extent at very low salt concentrations. With the addition of kosmotropic acetate (Ac−), hydration of the brushes increases, accompanied by a swelling of the brushes in the low‐salt‐concentration regime. In contrast, the brushes dehydrate and collapse with increasing concentration of Ac− in the high‐salt‐concentration regime. The counterion specificity of the brushes demonstrated here is determined by specific ion‐pairing interactions through modulating the osmotic pressure within the brushes and the hydrophobicity of the ion pairs.