As a phytochemical derived from the roots of Salvia miltiorrhiza Bunge, Tanshinone IIA has been reported to possess anti-inflammatory and antioxidant activity. Studies in breast, colon, prostate and lung cancer indicate that Tanshinone IIA may exhibit a promising antitumor activity. However, systemic studies of the cytotoxic effects of Tanshinone IIA on gastric cancer have not been described. The present study offers a comprehensive evaluation of the antitumor effects of Tanshinone IIA in gastric cancer cells in vitro and in a mouse xenograft model. Cell viability and apoptosis in vitro were evaluated through the MTT assay and flow cytometry analysis. The results indicate that Tanshinone IIA can induce gastric cancer cell growth inhibition and apoptosis in a time- and concentration-dependent manner. Furthermore, we investigated the mechanism of the apoptotic effects induced by Tanshinone IIA. We found that Tanshinone IIA can not only cause cell cycle arrest in the G2/M phase, but also trigger the intrinsic apoptotic signaling pathway. The results suggest that Tanshinone IIA may serve as an effective adjunctive reagent in the treatment of gastric cancer.