B7-H3 belongs to the B7 superfamily, a group of molecules that costimulate or downmodulate T cell responses. Although it has been shown that B7-H3 can inhibit T cell responses, several studies, most of them performed in murine systems, found B7-H3 to act in a co-stimulatory manner. In addition, B7-H3 is also expressed in various human cancers and is correlated with the poor outcome of cancer patients. The functional role of B7-H3 in cancer is still controversially discussed. In the present study, we compared B7-H3 expression in normal gastric tissues and gastric cancer tissue specimens and determined the effects of low B7-H3 expression on the human gastric cancer cell line SGC-7901 by using RNAi. B7-H3 expression in gastric specimens was determined by tissue qPCR and immunohistochemisty. A SGC-7901 cell line with low B7-H3 expression was established by lentiviral-mediated RNA interference to investigate the effect of B7-H3 on cancer cell migration and invasion in vitro. By establishing an orthotopic transplantation gastric cancer mouse model, the effect of B7-H3 on cell migration and invasion was studied in vivo. B7-H3 expression was significantly higher in the gastric cancer group than that in the normal gaster group. B7-H3 knockdown by RNA interference decreased cell migration and Transwell invasion up to 50% in vitro. In the orthotopic transplantation gastric cancer mouse model, the effect of inhibiting metastasis by knockdown of B7-H3 was assessed in terms of the average postmortem abdominal visceral metastatic tumor weight. The results revealed that inhibition of B7-H3 expression reduced gastric cancer metastasis in vivo. In conclusion, B7-H3 is aberrantly expressed in gastric cancer. In addition to modulating tumor immunity, B7-H3 may have a novel role in regulating SGC-7901 cell metastasis.