Monascus purpureus is a filamentous fungus traditionally used in East Asian cultures to produce red yeast rice (MpRYR), which contains beneficial compounds such as natural pigments and monacolin K. These bioactive components are sensitive to environmental factors including pH, heat, and oxidation. This study aimed to evaluate the nutritional composition and pH-related stability of the ethanol extract of MpRYR. The extract was prepared by macerating fermented rice with 70% ethanol, yielding a thick extract with a 9.43% extraction efficiency. Nutritional content was analyzed using standard proximate methods (SNI 01-2891-1992), while vitamin C was determined via iodimetric titration method. For stability pH analysis, the extract was mixed with varying volumes of 0.1 N HCl or NaOH, and absorbance was monitored at 0, 1, 2, and 3 hours using UV-Vis spectrophotometry (λ200–800 nm). The extract contained water (29.10%), carbohydrates (49.73%), protein (17.79%), lipids (2.13%), ash (1.25%), and vitamin C (405.86 mg/100 g). The pH stability changed slightly from 5.75 to 5.97 over 3 hours, with only 3.8% degradation, indicating strong stability under acidic conditions. These findings highlight the potential of MpRYR ethanol extract as a nutritionally rich and acid-stable ingredient for functional foods or medicinal use.