Fimasartan potassium trihydrate is chemically, 2-butyl-5-dimethyl amino thiocarbonyl methyl-6-methyl-3-[2'-(1H-tetrazol-5-yl) biphenyl-4-yl] methyl] pyrimidin-4(3H)-one potassium trihydrate. Precise and commercial UV-Spectrophotometric methods have been developed using water as a solvent. All five methods of UV-Spectrophotometry based upon Zero Order and First Order derivative Spectrophotometry have been established considering amplitude and Area under Curve of the spectrum. Fimasartan exhibits maximum absorbance at 261 nm when dissolved in water. In all four methods, Fimasartan obeyed linearity in the concentration range of 3-21 µg/mL with correlation coefficient (r2 >0.999). The % amount of drug estimated in the developed methods was found to be good agreement with label claimed in in-house tablet formulation. All the methods were validated as per International council on Harmonization (ICH) guidelines. All these proposed methods were proved to be linear, accurate, precise and rugged and also adequately sensitive.