Hexavalent chromium (CrVI) is much more toxic and mobile than the trivalent species (CrIII) and consequently, in situ monitoring of CrVI can improve the understanding of Cr biogeochemistry and toxicity in ecosystems. The passive diffusive gradients in thin-films (DGT) technique is a powerful tool for determining metal(loid) speciation, but a binding phase that absorbs only one specific species of Cr is needed. N-Methyl-d-glucamine (NMDG) functional resin was incorporated into the DGT binding phase for selective measurement of CrVI. This NMDG-DGT sampler exhibited a theoretically linear accumulation of CrVI, with negligible accumulation (<5%) of CrIII, even after 72 h deployment. The good prediction of CrVI concentration in synthetic freshwater with NMDG-DGT, even in the presence of 10-time more CrIII, further indicated the sampler’s reliability in selective detection of CrVI. Moreover, its high capacity for CrVI, which exceeded 230 μg cm–2, facilitates measurement of CrVI in both uncontaminated natural waters and in slightly and heavily contaminated (ppm level) waters. Field deployment of the NMDG-DGT sampler in such waters allowed accurate measurement of time-averaged CrVI concentration, indicating its robustness for in situ measurements of Cr speciation and its potential for further application in the risk assessment of Cr.