Here, we have developed a novel approach to the visual detection of molybdate with high sensitivity and selectivity in aqueous media based on the combination of catalytic formation of iodine and iodine-mediated etching of gold nanorods. In weak acid solution, like peroxidase, molybdate can catalyze the reaction between H2O2 and I- to produce I-2, a moderate oxidant, which then etches gold nanorods preferentially along the longitudinal direction in the presence of hexadecyltrimethylammonium bromide. The etching results in the longitudinal localized surface plasmon resonance extinction peak shifts to short wavelength, accompanied by a color change from blue to red. Under optimal conditions, this sensor exhibits good sensitivity with a detection limit of 1.0 nM. The approach is highlighted by its high selectivity and tolerance to interference, which enables the sensor to detect molybdate directly in real samples, such as tap water, drinking water, and seawater. In addition, perhaps the proposed sensing strategy can be also used for other targets that can selectively regulate the formation of I-2 under given conditions.