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A fast and selective colorimetric sensor for detecting Fe3+ was developed using gold nanoparticles capped with ortho-hydroxybenzoic acid (o-HBA AuNPs). This sensor was characterized using UV-Vis spectroscopy, FTIR spectroscopy, XRD, and TEM. The absorbance of surface plasmon resonance (SPR) at 534 nm increased after the addition of Fe3+. It was confirmed by FTIR, XRD, and TEM data indicating that Fe3+ acted as a bridge among the capping agents of AuNPs. The bridges blocked AuNPs from aggregation and caused an increase in the uniformity distribution of o-HBA AuNPs. Under the optimized condition, this sensor had a good calibration curve linearity at 534 nm. The value of R2 obtained was 0.9994 with LoD of 9.19 μM and LoQ of 30.57 μM. This sensor exhibited good precision and accuracy for Fe3+ detection in rice field, river, and sea waters and has been successfully used to detect Fe3+ in the real water samples collected from various locations of Code River, Yogyakarta, Indonesia. [Display omitted] • An o-HBA AuNPs-based colorimetric sensor is synthesized and used to detect Fe3+. • The sensor is highly selective towards Fe3+. • The sensor is successfully used to determine Fe3+ in rice field, river, and seawater. [ABSTRACT FROM AUTHOR]