Previous studies reported that the aberrant expression of miR-19b in gastric cancer tissues and circulating miR-19b is a potential biomarker to indicate progression of gastric cancer. However, the prognostic significance of miR-19b, and its role and underlying mechanisms in gastric cancer remain poorly investigated. In the present study, we demonstrated that the expression of miR-19b was aberrantly downregulated in both gastric cancer tissues and cell lines. Clinical association analyses disclosed that the reduced expression of miR-19b was significantly associated with adverse clinicopathological characteristics including poor differentiation, large tumor size and advanced tumor-node-metastasis (TNM) stage. Gastric cancer patients with low expression of miR-19b had prominent shorter overall survival and disease-free survival. Gain-of-function studies indicated that miR-19b overexpression inhibited cell proliferation and cell cycle progression in MGC-803 cells. While miR-19b silencing promoted cell proliferation and cell cycle progression in SGC-7901 cells. Furthermore, in vivo experiments showed that miR-19b overexpression suppressed the tumor growth of MGC-803 cells. Notably, miR-19b inversely regulated B-cell CLL/lymphoma 3 (BCL3) abundance in gastric cancer cells. BCL3 was identified as a direct target of miR-19b using luciferase reporter assays. Moreover, BCL3 knockdown abolished the effects of miR-19b knockdown on gastric cancer cells. In conclusion, our data suggest that miR-19b may potentially serve as a novel prognostic biomarker and therapeutic target for gastric cancer.