Relative Energy Deficiency in Sport (RED-S) has emerged as a critical multidimensional health concern among female athletes, affecting metabolic, endocrine, skeletal, psychological, and performance-related systems. This review aimed to synthesize current scientific evidence on RED-S and integrate recent real-world case reports to provide a comprehensive understanding of its clinical presentation, underlying mechanisms, and management strategies. A systematic literature search covering 2020–2025 was conducted across PubMed, Scopus, Web of Science, SPORTDiscus, and Google Scholar using keywords related to RED-S, low energy availability (LEA), female athletes, menstrual dysfunction, and bone health. Eligible studies included randomized controlled trials, observational studies, systematic reviews, consensus statements, and published case reports. Findings demonstrate that LEA (<30 kcal/kg FFM/day) is the primary driver of RED-S, leading to metabolic suppression, impaired thyroid and gonadal function, and disruptions in key anabolic pathways. Menstrual dysfunction ranging from luteal phase defects to functional hypothalamic amenorrhea emerged as a sensitive early indicator of RED-S. Literature and case reports consistently show that restoration of energy intake results in gradual endocrine recovery, with menstruation returning within approximately 2–12 months depending on severity. Bone health consequences were substantial, with reduced bone mineral density and elevated stress-fracture incidence particularly evident in endurance and aesthetic sports. Performance decrements were observed across VO₂max, lactate threshold, sprint capacity, and recovery metrics. Psychosocial determinants including perfectionism, body-image pressure, disordered eating tendencies, and coach–athlete communication were strongly associated with RED-S risk. Current evidence supports multidisciplinary management emphasizing nutritional rehabilitation, psychological support, training-load modification, and routine screening using updated IOC REDs CAT2 guidelines.