An accurate method was developed for the determination of Brilliant Blue FCF, a synthetic soluble colorant, in environmental samples. The method is based on the catalytic effect of silver nanoparticles (AgNPs) on the oxidation of Brilliant Blue FCF by hexacyanoferrate(III) in acetate–acetic acid medium at room temperature. Linearity of around four orders in the magnitude of concentration was generally obtained. Detection and quantification limits of the investigated dye, which was evaluated at a signal-to-noise ratio of 3 for detection limit and quantification limit, were 0.1 and 0.6–130.0 ng/mL, respectively. The recoveries of the synthetic colorant Brilliant Blue FCF in environmental matrices (surface water) ranged from 95.0 to 101.0%. Relative standard deviation of less than 3.1% was also achieved. This method has been applied successfully in the determination of water-soluble colorant Brilliant Blue FCF in surface water samples.