Abstract: Microribonucleic acids (miRNAs) are 20 nt to 25 nt non-coding RNAs that are widely distributed in eukaryotes. MiRNAs regulate gene expression in a sequence-specific manner and regulate diverse expression patterns that are dependent on cell status and identity. MiRNAs function as gene regulators, controlling many cellular processes such as cell growth, differentiation, proliferation, and apoptosis. MiRNA expression is also reportedly correlated with many cancers, by acting as oncogenes or tumor suppressors. MiR-21 is overexpressed in many kinds of tumors, including human lymphomas. MiR-21 promotes tumorigenesis and affects tumor progression and prognosis through a variety of ways by influencing tumor suppressor genes and related signaling pathways. MiR-21 silencing using an antisense nucleotide increases the sensitivity of lymphoma cells to chemotherapy and inhibits the growth of tumor cells. Therefore, studying miR-21 may provide new insights into the pathogenesis and treatment of lymphoma. In this review, the authors summarized the current knowledge on the significance of miR-21 in human lymphomas, with an emphasis on its regulation, mechanism of action, target sites in human lymphoma cells, and its potential as a disease biomarker and novel therapeutic target.