This method involves development of a highly sensitive spectrophotometric procedure to estimate mesalazine as pure substance and in the tablets. This particular procedure was focused on the diazotization of mesalazine with an excess quantity of sodium nitrite (NaNO2) in an acid medium using HCl solution to produce a corresponding diazonium compound which reacts with 8-hydroxyquinoline reagent in an alkaline solution of NaOH to yield read-orange azo dye which is soluble in water and showed maximum absorption peak at the wavelength of 500 nm against the blank solution. The calibration graph was linear and compatible to Beer's law over the concentration range from 0.25 to 12.5 μg/ml with an exceptional determination coefficient (R2= 0.9994) and apparent molar absorptivity 2.88 × 104 L.mol-1cm-1. The limits of detection (LOD) and quantitation (LOQ) were premeditated and found to be 0.2023 and 0.6524 μg/ml, correspondingly. A relative error percent (accuracy) and the relative standard deviation (RSD%) was also calculated and found to be in the range -3.84% - 2.70%, and 0.17% - 1.94%, correspondingly. No interferences were observed from other ingredients that may be exist in the tablets. The stoichiometry of the resulting azo dye has been examined and the experimental results revealed that the mole ratio of mesalazine to 8-hydroxyquinoline is 1:1. The advised procedure was successfully applied for the determination of mesalazine in its pharmaceutical form (tablets).