Selenium is an essential trace nutrient for the human body, playing a crucial role in maintaining redox homeostasis, immune function, and cellular homeostasis. With the rapid development and interdisciplinary convergence of nutritional epidemiology, tumor biology, and bioinorganic chemistry, the potential role of selenium in tumorigenesis, progression, and prognosis has garnered increasing attention. Extensive epidemiological, animal experimental, and clinical intervention studies indicate that selenium metabolic homeostasis or dietary intake levels are closely associated with the risk of developing various cancers, disease progression, and patient prognosis. However, results vary across different populations, tumor types, and study designs, exhibiting dose-dependent effects, tumor specificity, and individual heterogeneity. This systematic review summarizes recent epidemiological and clinical research progress on selenium and common tumors, and summarizes the relationship between selenium exposure levels and tumor incidence risk and prognosis in different tumor types. It aims to provide theoretical reference for the scientific application of selenium in tumor prevention and treatment, and to provide theoretical basis for future large-scale population and clinical intervention studies.