Metformin as the first-line treatment for type 2 diabetes mellitus has been discovered to exert beneficial effects on many diseases for nearly ten years, but its specific mechanism is still unclear. As a new class of gene expression regulators with pleiotropic properties, microRNAs (miRNAs) participate in multiple physiological processes such as cell differentiation, proliferation, survival, and metabolism, which drive them to play a regulatory role in the occurrence, development and even treatment of various diseases. A substantial body of research has found the relationship between metformin and miRNAs, in which metformin can alter the expression profiles of miRNAs in multiple disease states and on the other hand the signal pathways involving miRNAs may contribute to the pharmacological actions of metformin. This review summarizes the effects of metformin on miRNAs and their relationship in different diseases (like tumor, metabolic diseases, etc.), which should be of a great help for our better understanding of the mechanism of metformin for treating multiple diseases.