Selenium, an essential trace element, has garnered significant attention for its multifaceted role in cancer biology, particularly in breast cancer. As a component of selenoproteins, selenium contributes to antioxidant defense, immune modulation, and apoptosis regulation, making it a promising candidate for cancer prevention and therapy. Epidemiological and experimental studies suggest that selenium supplementation may reduce oxidative stress, enhance immune surveillance, and suppress tumor initiation in breast cancer. These protective effects underscore selenium's potential as an ally in combating breast cancer. However, the role of selenium is far from straightforward, as excessive selenium intake or specific genetic contexts can lead to pro-oxidant effects, promoting DNA damage, inflammation, and tumor progression. Moreover, selenium has been implicated in chemoresistance, potentially hindering the efficacy of standard breast cancer treatments. This dual nature, often referred to as the "selenium paradox," complicates its therapeutic application. The variability in selenium's impact, influenced by genetic polymorphisms, baseline selenium status, and the tumor microenvironment, underscores the need for a nuanced understanding of its role in breast cancer.