An incredible surge in demand for fermented dairy products has been experienced recently due to its nutraceutical benefits and lifestyle preferences. As a traditional fermented dairy beverage, cultured buttermilk is an ideal product choice in this context. This study introduces a novel cultured buttermilk fortified with the high protein microalgae Arthrospira platensis, making it beneficial and appealing due to its preferable mouthfeel and antioxidant characteristics. The goal of this study was to extensively evaluate and determine the optimum amount of Spirulina powder to be mixed into buttermilk, while also analyzing the alterations in the physicochemical and sensory properties of the resulting product. The pH, titratable acidity, DPPH inhibition activity, Phenolic content, and sensory properties of newly developed cultured buttermilk with varying concentrations of Spirulina 0.25, 0.45, 0.65, 0.85 and 1.5% were compared with that of the control buttermilk sample with no added Spirulina. Based on the findings derived from the conducted study, it has been concluded that the incorporation of 0.25% Spirulina, can potentially enhance the nutritional composition and functional health attributes of cultured buttermilk without affecting sensory score Keywords: Butter milk, fortification, Arthospira platensis, antioxidant activity, total phenolic content, sensory evaluation