MicroRNAs (miRNAs) are small non-coding RNA molecules that diversely regulate physiological and pathophysiological processes by specifically binding to different regions of targeting messenger RNAs (mRNAs). Fibrosis is characterized by the abnormal proliferation of fibroblasts and the deposition of the extracellular matrix (ECM). Both clinical and experimental animal studies have revealed that aberrant expression of miRNAs is closely associated with the development of fibrotic diseases. microRNA-21 (miR-21) is a ubiquitously expressed miRNA that is traditionally considered to be an oncogenic miRNA (oncomiR). Recent studies have demonstrated that elevated expression of miR-21 may play a vital role in the development of fibrosis by promoting the proliferation of interstitial fibroblasts and increasing the abnormal deposition of the ECM. In this review, we comprehensively summarize the role of miR-21 in tissue fibrosis. Furthermore, we highlight miR-21 as a potential diagnostic and prognostic marker and therapeutic target for fibrosis diseases.