Lansoprazole (LAN) is an important proton pump inhibitor that suppresses gastric acid secretion. A simple and rapid titrimetric method has been developed for the determination of LAN in pure drug and in its capsule formulation. The method is based on the neutralization of basic amino group in LAN with perchloric acid as titrant in 1,4-dioxane medium. The end point was detected using methyl red as indicator with the color change from yellow to red. Method is applicable over 2 – 20 mg range and the calculations are based on the molar ratio of 1: 1 (LAN: HClO4). The method was statistically evaluated by calculating percent relative error (%RE) for accuracy and percent relative standard deviation (% RSD) for precision, and were applied successfully to the determination of LAN in capsules. No interference was observed from common additives found in pharmaceutical capsules. The accuracy and reliability of the method were further ascertained by performing recovery tests via standard-addition technique.