This study aimed to develop a probiotic fermented beverage inspired by the traditional Brazilian indigenous beverage Cauim. The beverage was formulated using peanuts and soybeans and fermented using a combination of bacteria and yeast based on traditional methods of Brazilian indigenous cultures. The composition and processing conditions of the beverage were optimized using a Plackett-Burman design and response surface methodology. Strains isolated from indigenous Brazilian fermentations, Pediococcus acidilactici CCMA 0347, and Saccharomyces cerevisiae CCMA 0681, were used in addition to the commercial probiotic Lactobacillus acidophilus (HOWARU). The optimized formulation contained 61.2% water-soluble peanut extract, inoculated with 8.00, 6.70, and 5.0 log CFU/ml of , and , respectively. The fermented beverage maintained a high level of probiotic cell viability (>10 CFU/ml) over 35 days of storage. The metabolic activity of the microorganisms was sustained throughout storage, mirroring the behavior observed in traditional fermented foods such as Cauim. In addition to high probiotic viability, the beverage presented elevated levels of lactic acid and low pH, indicating the presence of postbiotic components. It also showed nutritional richness in proteins, isoflavones, unsaturated fatty acids, and micronutrients. Sensory analysis demonstrated good acceptance, particularly in appearance and aroma, although further improvements are needed in flavor perception. These results indicate that the optimized plant-based beverage is a promising functional food with probiotic and postbiotic properties, good sensory appeal, and potential to promote health while enhancing the cultural value of Brazilian Indigenous food traditions.