A novel humanized IgG1 monoclonal antibody known as Lecanemab is used to treat Alzheimer’s disease. Utilizing the RP-UPLC technique, the current investigation aims to develop a robust, stable and rapid method for determining Lecanemab. By applying this method, the ultimate chromatographic conditions were obtained using a BEH C8 (1.7 μm, 2.1 x 100 mm) and a mobile phase prepared with acetonitrile and formic acid buffer in a 7: 3 ratio, respectively (1.0 mL of formic acid diluted in 1 liter of HPLC-grade water), by means of adopting isocratic elution. Detection limit (LOD) and quantitation limit (LOQ) were 0.30 and 1.00 μg/mL, respectively, and linearity (R2 = 0.999) was demonstrated in the range of 25-150 μg/mL of working concentration with PDA detection at 219 nm. The results of the forced degradation investigation also make it evidently visible; the drug’s degradation products could be separated from the main peak, assuring the effectiveness of the stability-indicating method. Thus, routine analysis and stability studies favor of enormously from the use of this method of analysis.