Cullin 4B (CUL4B) is a scaffold protein overexpressed in several solid malignancies. It is known to silence tumor suppressor through post-transcriptional manner. However, its clinical significance and underlying molecular mechanisms in gastric cancer (GC) remain largely unknown. In this study, we found that CUL4B was significantly overexpressed in GC tissues and its overexpression was correlated with lymph node metastasis and poor prognosis. Through gain- and loss-of-function experiments, we showed that CUL4B promotes GC cell invasion and epithelial-mesenchymal transition (EMT) in vitro, as well as tumor growth and metastasis in vivo. Mechanistically, we identified HER2 as a downstream target gene of CUL4B in GC. CUL4B unregulated HER2 expression via transcriptionally repressing miR-125a. Intriguingly, HER2 inhibitors significantly reversed CUL4B-induced EMT in vitro and partially blocked GC metastasis in tumor xenografts with CUL4B overexpression. Finally, we suggested the involvement of the PI3K/AKT pathway in CUL4B-induced HER2 upregulation in GC. In all, we proposed a model for a CUL4B-miR-125a-HER2 oncoprotein axis, which provided novel insight into how HER2 was activated and contributed to GC progression and metastasis.