Opuntia ficus-indica flour supplementation at 15% concentration significantly improved memory performance in elderly rats, with treated animals showing enhanced recognition and discrimination indices in both short- and long-term cognitive tests. The intervention increased brain glutathione levels, reduced protein carbonyls, and elevated beneficial fatty acids including docosahexaenoic acid and arachidonic acid. Crucially, the 15% dose restored intestinal microbial diversity and enriched short-chain fatty acid-producing genera like Faecalibaculum and Dorea. This research advances our understanding of the gut-brain axis in aging by demonstrating how a specific plant-based intervention can simultaneously target multiple pathways—antioxidant defenses, neuroinflammation, and microbiome composition. The findings are particularly relevant given the growing recognition that cognitive decline often correlates with gut dysbiosis in older adults. While promising, these results require human validation, as rodent microbiomes differ substantially from human gut ecosystems. The dose-dependent effects suggest an optimal therapeutic window, with the highest concentration providing the most comprehensive benefits across cognitive, oxidative, and microbial parameters.