Background/Objectives: Phosphorus plays a fundamental role in cellular and extracellular metabolism, contributing to nucleic acid synthesis, enzymatic activity, neurologic function, and skeletal mineralization. Despite its significance, non-severe hypophosphatemia (HP) remains largely asymptomatic and underdiagnosed, with limited data on its prevalence in the general population. Most studies focus on specific subgroups, such as critically ill or dialysis patients, while the impact of mild HP in older adults, a potentially vulnerable demographic, is not well understood. The objective of this systematic review is to investigate the prevalence and clinical implications of non-severe HP in older adults. Methods: The study followed PRISMA guidelines to assess HP in patients aged ≥ 65 years without critical illnesses or genetic disorders. A systematic search was conducted in PubMed, Web of Science, and Scopus (March 2024). Eligible studies included RCTs and prospective/retrospective studies, excluding cancer-related studies or insufficient phosphate data. Results: We identified 1350 articles, with 26 meeting eligibility criteria. Ultimately, eight studies involving 26,548 patients were included, with an HP prevalence of 12.5%. Studies reveal a higher prevalence of HP in frail individuals, particularly those with increased frailty scores, and an association between HP and cognitive decline, depressed mood, falls, and chronic comorbidities. HP was also prevalent in infectious diseases, especially bacterial pneumonia, with longer hospital stays and increased mortality rates. Malnutrition was significantly more common in HP patients, characterized by weight loss and poor nutritional status. Furthermore, HP increased fall risk during hospitalization and worsened outcomes after coronary artery bypass graft surgery, including higher mortality and graft failure rates, underscoring its clinical importance. Discussion: This review identified that non-severe hypophosphatemia (HP) is associated with conditions such as frailty, cognitive decline, and an increased risk of falls. The evidence suggests that low phosphate levels may negatively impact health, increasing mortality and the risk of adverse clinical outcomes. Despite limitations in diagnostic criteria and sample variability, the findings indicate that HP can be a useful marker for identifying patients at risk of health deterioration. Further research is needed to clearly define the management and treatment of HP in this vulnerable population.