Background: The current work, which was conducted under the quality by design (QbD) paradigm, aims to establish the optimization of ultra-performance liquid chromatography (UPLC) using the design of experiments and response surface theory, such as central composite design (CCD), in order to achieve a good separation and resolution. Methods: The mobile phase was composed of methanol (20:80% v/v) and 0.1% tri fluro amine (TFA) buffer, and chromatographic separation was performed on column phenomenex C18 (50 mm x 4.6 mm x 2.5 m) at a flow rate of 0.4 mL/ min. Montelukast (MON) and bilastine (BIL) were both detected at 260 nm. The suggested method was approved in accordance with International Council for Harmonization (ICH) recommendations. Results: The created technique successfully separates both BIL and MON with a chromatographic resolution of 6.4. The technique was linear for BIL and MON concentrations of 0.5–3.0 μg/mL and 0.25–1.5, respectively, and recovery rates ranged from 99–100%. The drug compounds had distinct degradation products that were identified in stress patterns. Conclusion: For the quantification of BIL and MON in the combination tablet, a precise, simple, reliable, and accurate UPLC method was designed. According to the method validation, it was possible to successfully separate two medicines from their degradants utilizing aQbD techniques.