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A new spectrophotometric method is reported for the determination of nanomolar level of malachite green in surface water samples. The method is based on the catalytic effect of silver nanoparticles on the oxidation of malachite green by hexacyanoferrate (III) in acetate–acetic acid medium. The absorbance is measured at 610 nm with the fixed-time method. Under the optimum conditions, the linear range was 8.0 × 10⁻⁹–2.0 × 10⁻⁷ mol L⁻¹ malachite green with a correlation coefficient of 0.996. The limit of detection (S/N = 3) was 2.0 × 10⁻⁹ mol L⁻¹. Relative standard deviation for ten replicate determinations of 1.0 × 10⁻⁸ mol L⁻¹ malachite green was 1.86 %. The method is featured with good accuracy and reproducibility for malachite green determination in surface water samples without any pre-concentration and separation step.