Imatinib (STI-571, Gleevec, Glivec, CGP 57148) is a protein-tyrosine kinase inhibitor that targets the ABL family of proteins. Imatinib is a drug that is used to treat some kinds of leukemia, as well as other malignancies and blood cell diseases. Imatinib also treats several forms of gastrointestinal stromal tumors (GIST). This study presents a new Zero-order Absorbance UV-Spectrophotometric approach for determining Imatinib Mesylate in commercial tablets and during dissolution tests that is simple, easy, accurate, precise, economical, and sensitive. It was possible to determine the concentration of Imatinib Mesylate in commercial tablets using 0.1N HCl at a λmax of 234 nm in a linear range of 3-18 g/mL with an R2> 0.99 and recovery between 99.27 and 99.72 %. The % amount of drug estimated in the developed method was found to be in good agreement with the label claimed in commercial tablet formulation. The technique was also optimized for quantifying Imatinib mesylate in dissolution tests in the same linear concentration range. This procedure has the advantage that it allows direct measurement of samples from the dissolution vessel without any pH correction. Further, the method has been validated for accuracy, precision, sensitivity, and ruggedness as per ICH guidelines.