Type 2 diabetes mellitus (T2DM) is a serious health problem, and the number of affected persons is increasing annually worldwide. Obesity is the main cause of diabetes. As an individual gain weight, adipocytes are observed to become bigger. Hypotrophic adipocytes shows decreased adiponectin secretion and increased free fatty acids and inflammatory adipokines, which leads to lowered insulin sensitivity and metabolic activity. In contrast, having smaller and fewer adipocytes lead to increases in insulin sensitivity and adiponectin secretion and decreases in the release of inflammatory cytokines. Therefore, increasing the number of smaller adipocytes has been suggested as an effective strategy to prevent and treatment of T2DM. Although thiazolidinediones, such as troglitazone and pioglitazone, which are used currently to treat T2DM, can improve insulin sensitivity by increasing the number of small adipocytes, the side effects of these drugs include weight gain and liver failure. As an alternative treatment for T2DM, polyphenolic compounds, such as nobiletin and sakuranetin, reportedly induce preadipocytes to become small adipocytes. In addition, our recent study showed that the polyphenol cyanidin-3-glucoside (Cy3G), an anthocyanin, similarly induces 3T3-L1 preadipocytes to become small adipocytes, and several other researchers have obtained diverse evidence that supports the efficacy of Cy3G in the prevention or treatment of T2DM. Here, we summarize the activities of Cy3G that may support its use in the prevention of T2DM, focusing on the drug’s effect on adipocytes.