Gastric cancer (GC) is one of the most common cancers and the leading cause of cancer-related deaths globally. The discovery of microRNAs (miRNAs) provides a new avenue for GC diagnostic and treatment regiments. Currently, a large number of miRNAs have been reported to be associated with the progression of GC, among which miR-378 has been examined to be downregulated in GC tissues and several cell lines. However, the function of miR-378 on GC cells and the mechanisms were less known. Here we found that ectopic expression of miR-378 could inhibit cell proliferation, cell cycle progression, cell migration as well as invasion, and induced cell apoptosis in GC cell line MGC-803. Moreover, we found that oncogene mitogen-activated protein kinase 1 (MAPK1) was a target gene of miR-378 in GC cells, and the tumor-suppressive role of miR-378 might be achieved by the direct interaction with MAPK1. Taken together, our results showed that miR-378 might act as tumor suppressors in GC, and it may provide novel diagnostic and therapeutic options for human GC clinical operation in the future.