Two simple and sensitive spectrophotometric methods (A and B) for the assay of Darunavir (DNV), in pure and pharmaceutical formulations were developed. The method-A is based on the condensation reaction of DNV forming an orange-red colored Schiff's base of maximum absorption peak (λ max) at 444 nm with sodium 1,2-naphthaquinone-4-sulfonate (NQS) in an alkaline medium. The dimethoxy benzene nucleus of brucine is attacked by IO4- with the formation of O-quinone (Bruci quinone) which in turn undergoes nucleophilic attack on the most electron rich position of the coupler i.e., proton bearing amino group (primary amine of DNV), to give 1-mono substituted Bruci quinone derivative. The reaction mechanism in both methods were discussed. The absorbance of colored complex is found at 511nm. Beer's law is obeyed in the concentration range 10-60 μg/ml, 50-300 μg/ml, the Molar absorptivity values are1.3874x 105, 2.9865x 104 L/mol.cm, Correlation Coefficients are 0.9988,0.9996, Sandell sensitivity are 3.9473x 10-3, 1.8337 x 10-2ng/cm2 .The methods proposed gave reproducible results with the percentage recoveries in the formulations found to be 99.812 to 99.749 and 99.682 to 99.774 for Methods A and B.