This study involves a development method for spectrophotometric analysis of sulfamethoxazole (SMZ) with vanillin reagent in acidic medium. The reaction mechanism was proposed by Schiff's base reaction of the amine group in SMZ with the carbonyl group in vanillin. The spectrum of SMZ complex showed high absorbance at 399.09 nm following the Beer–Lambert law in the concentration levels ranging from 5 to 80 µg.mL-1 with good sensitivity. The stability constant of the product was determined. The values of relative standard deviation (≤ 6.63) and recovery (> 98.73%) indicated a reasonably precise and accurate method with a good limit of detection and quantification. The present method was compared with other spectrophotometric methods in the literature. The procedure of the method showed to be simple since no need for temperature control and solvent extraction. The suggested method was used to determine of SMZ in commercial pharmaceutical drops and compared with the certified method.