In the current study, we present a colorimetric sensing method for Cr(iii) in aqueous solution using synthesized gold nanoparticles (AuNPs) without any surface functionalization. The method is based on the aggregation of citrate capped AuNPs by Cr(iii) leading to a red-shift of the UV-visible absorption maxima from 526 nm to 714 nm. Dynamic light scattering measurements and scanning electron microscopy confirmed the aggregation of the AuNPs. Under the optimized experimental conditions, a linear relationship (correlation coefficient r = 0.997) was established between the ratio of the absorbance at 714 nm to that at 526 nm (A714/526) and the concentrations of Cr(iii) over a range of 10-3 to 10-6 M [50000-50 μg L-1]. The reported probe had a limit of detection of 1.06 × 10-7 M [5.3 μg L-1] and demonstrated good selectivity towards Cr(iii). The applicability of the technique was extended to a simple paper based assay, wherein the AuNPs immobilized on Whatman filter paper strips interacted with Cr(iii) and the color intensity measured was calibrated against the analyte concentrations. Under the optimized conditions, a linear relationship (correlation coefficient r = 0.99) was noted between the measured color intensity and the concentration of Cr(iii) within the range of concentrations 10-3 to 10-6 M and the theoretical limit of detection was computed to be 1.53 × 10-7 M. The paper strip based method possessed high selectivity towards Cr(iii), and was also successfully tested against an artificial mixture of interfering metal ions. The current study is the first of its kind to report the development of a paper strip based assay for the colorimetric sensing of chromium using as synthesized AuNPs without any surface functionalization, and may facilitate the on-site monitoring of Cr(iii) in contaminated sites. © 2013 The Royal Society of Chemistry.