MicroRNA-21 (miR-21) is a key regulator of gene expression involved in cancer progression and fibrosis. Its dysregulation promotes cell proliferation, resistance to apoptosis, Epithelial-Mesenchymal Transition (EMT), and Extracellular Matrix (ECM) remodeling, making it one of the most widely studied oncomiRs and fibromiRs. Evidence shows that miR-21 contributes to tumor growth, metastasis, and fibrosis by suppressing tumor suppressors such as PTEN and PDCD4 and activating pro-fibrotic pathways, including TGF-β signaling. Its consistent dysregulation highlights its potential as a biomarker for diagnosis, prognosis, and treatment monitoring. Several therapeutic strategies, such as antagomiRs, locked nucleic acids (LNAs), CRISPR-based inhibition, and nanoparticle- or viral-mediated delivery systems, have shown promising preclinical results. Despite advances, challenges remain in translating miR-21 inhibition to clinical use. Key limitations include off-target effects, delivery inefficiencies, and context-dependent variability in miR-21 function. These issues hinder the development of safe and effective miR-21-targeted therapies. MiR-21 plays a central role in cancer and fibrosis and offers significant potential as both a biomarker and therapeutic target. Future research should focus on precision-based approaches, next-generation gene-editing technologies, and improved preclinical models to optimize miR-21 inhibition for clinical application. This review compiles and evaluates current literature on miR-21 biogenesis, regulation, functional roles, and therapeutic targeting across various cancers and organ-specific fibrotic disorders.