Genistein (4',5,7-trihydroxyisoflavone) is a phytoestrogen belonging to a subclass of natural flavonoids that exhibits a wide range of pharmacological functions, including antioxidant and anti-inflammatory properties. These characteristics make genistein a valuable phytochemical compound for the prevention and/or treatment of cancer. Genistein effectively inhibits tumor growth and dissemination by modulating key cellular mechanisms. This includes the suppression of angiogenesis, the inhibition of epithelial-mesenchymal transition, and the regulation of cancer stem cell proliferation. These effects are mediated through pivotal signaling pathways such as JAK/STAT, PI3K/Akt/mTOR, MAPK/ERK, NF-κB, and Wnt/β-catenin. Moreover, genistein interferes with the function of specific cyclin/CDK complexes and modulates the activation of Bcl-2/Bax and caspases, playing a critical role in halting tumor cell division and promoting apoptosis. The aim of this review is to discuss in detail the key cellular and molecular mechanisms underlying the pleiotropic anticancer effects of this flavonoid.