Brewer's spent grain (BSG) is an abundant by-product of the brewing industry, presenting both high value as a food raw material and an environmental challenge. Probiotics are widely recognized for their beneficial effects on human health and nutrition, particularly due to their role in the intestinal microbiota. Water kefir, a source of beneficial microorganisms, generates surplus granules that can be reused. This study aims to utilize these surpluses to produce a cereal bar based on BSG, cereals, and kefir granules (KGs), offering a high nutritional profile rich in proteins, fibers, and beneficial microorganisms while reducing costs within the framework of a circular economy. The BSG was dehydrated to less than 5% moisture, and three formulations were optimized with different percentages of BSG (20, 30 and 40%). BSG, whole grains, xanthan gum, kefir, sucralose and yogutrh thickener were the ingredients used. The content of the final product was evaluated for its nutritional profile. Proteins were determined by the official Kjeldahl AOAC 991.20 method (34.61 g per 100 g), fats by Soxhlet extraction (7.69 g per 100 g), ashes by calcination in a muffle furnace (2.3 g per 100 g), dietary fiber by gravimetric and enzymatic methods (20 g per 100 g), sodium by atomic absorption spectroscopy (40 mg) and carbohydrates by difference (31.15 g per 100 g). Total mesophiles, mycotoxins, fungi, and yeasts were analyzed in the industrially dehydrated BSG. KGs were dehydrated by low-temperature convection, and microbial viability was assessed by plate counts of lactic acid bacteria and yeasts. The cereal bar was formulated by combining BSG, KG, and cereals. The final product was evaluated for its nutritional content, shelf life, and sensory acceptability. The resulting product demonstrated high sensory acceptability, with significant fiber (30%), protein (40%), and probiotic microorganism content.