Diet is one of the primary determinants of gut microbiota composition and its metabolic output. That output is emerging as a critical determinant of systemic health and has been linked to most physiological systems in the body, yet its relationship to skeletal muscle physiology has only recently received attention. Evidence from experimental, observational, and clinical studies supports the existence of a gut–muscle axis, in which microbial composition and metabolic activity in the gut, influenced by diet, affect muscle metabolism, strength, function, and physical performance. Fiber, protein intake, probiotics, prebiotics, synbiotics, and fermented foods are examples of dietary constituents that transiently alter the gut microbiota, producing key metabolic mediators including short-chain fatty acids (SCFA), microbially generated amino acid products, and metabolites implicated in systemic inflammation and gut barrier integrity, alongside endocrine crosstalk involving myokines and hormones (e.g., cortisol and testosterone). Human studies consistently associate higher microbial diversity and enrichment of SCFA-producing taxa with improved muscle-related outcomes, while age-related dysbiosis has been linked to sarcopenia and functional decline. This narrative review summarizes current evidence of the gut–muscle axis and highlights potential mechanisms linking gut microbial composition to skeletal muscle function. Future advances in personalized nutrition may offer novel opportunities to preserve muscle health and functional capacity throughout aging.