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The muscle-brain axis integrates peripheral metabolic activity with central nervous system function. Among the endocrine signaling molecules regulating such crosstalk, the peptide hormone irisin released during muscle contraction seems to play relevant roles. Irisin is generated by the proteolytic cleavage of the fibronectin type III domain-containing protein 5 and has emerged as a key regulator of neurotrophic and metabolic adaptation. Although initially described as a myokine, irisin is also expressed in adipose and neural tissues, acting through autocrine, paracrine, and endocrine mechanisms. Irisin binds to the αV/β5 integrin receptor complex and activates a network of signaling pathways which promote mitochondrial biogenesis, autophagy, oxidative stress resistance, and modulation of inflammatory responses. Within the central nervous system, irisin induces brain-derived neurotrophic factor expression and contributes to synaptic plasticity, neurogenesis, and cognitive preservation. Experimental models show that irisin reduces amyloid burden, limits α-synuclein pathology, suppresses neuroinflammation, and stabilizes blood-brain barrier integrity, supporting a disease-modifying role in neurodegenerative conditions. In skeletal muscle, irisin stimulates myogenesis, enhances anabolic signaling, and improves metabolic efficiency, suggesting broader relevance for sarcopenia and age-related metabolic decline. Herein, we discuss irisin as a promising biomarker and a candidate therapeutic target for disorders characterized by concurrent muscle deterioration and cognitive impairment.