Purpose It is known that microRNAs (miRNAs) are a class of small, noncoding RNAs that act as key regulators in various physiological and pathological processes. However, the regulatory mechanisms involving miRNAs in retinoblastoma (RB) remain largely unknown. The miRNA miR-125b is dysregulated in various human cancers such as breast cancer, human hepatocellular carcinoma, ovarian cancer, and colorectal cancer. However, the significance of miR-125b in RB has not been sufficiently investigated. Our objective was to explore the role of the miR-125b in RB. Methods In this study, we measured miR-125b levels using real-time polymerase chain reaction in human RB cell lines, including HXO-Rb44, Y79, SO-RB50, and the normal human retinal pigment epithelial cell line ARPE-19; a total of 38 pairs of primary RB tissues and adjacent noncancerous tissues were also measured. In addition, overexpression of miR-125b in RB cell lines was performed to determine the role of miR-125b in RB. Results We found that miR-125b is significantly upregulated in RB, and closely associated with tumor cell proliferation and apoptosis. In addition, overexpression of miR-125b apparently promotes RB cell proliferation and migration in vitro. Gain-offunction in vitro experiments further showed that the miR-125b mimic significantly suppressed RB cell apoptosis. A subsequent dual-luciferase reporter assay identified the suppressor gene DRAM2 as direct target of miR-125b.Conclusions Our data collectively demonstrate that miR-125b is a suppressor gene miRNA that can promote RB cell proliferation and migration by downregulating the suppressor gene DRAM2, indicating that miR-125b may represent a new potential diagnostic and therapeutic target for RB treatment.