Enhancer of Zeste Homolog 2 (EZH2) is a key epigenetic regulator known for its role in global gene silencing and is involved in a variety of cellular processes, including cell survival, proliferation, invasion, and self-renewal. As a core component of the Polycomb Repressive Complex 2 (PRC2), EZH2 catalyzes the trimethylation of histone H3 at lysine 27 (H3K27me3), leading to chromatin compaction and transcriptional repression. Dysregulated EZH2 expression is observed in a wide range of solid tumors and hematological malignancies and is frequently associated with increased metastatic potential and poor clinical outcomes. While EZH2 primarily mediates gene silencing through its canonical PRC2-dependent activity, it also exerts oncogenic effects via non-canonical mechanisms. In its non-canonical role, EZH2 acts independently of PRC2, interacting with other signaling molecules as a transcriptional activator or co-activator, thereby promoting the activation of oncogenic pathways. Through both canonical and non-canonical mechanisms, EZH2 significantly contributes to tumor initiation and its subsequent progression. Given its critical role in oncogenesis and cancer progression, EZH2 is under investigation as a potential biomarker for cancer diagnosis and prognosis. This review provides a comprehensive overview of EZH2’s function and oncogenic roles across human cancers. Enhanced insight into EZH2’s complex regulatory network may facilitate the development of more effective strategies to manage EZH2-driven malignancies.