Tanshinone IIA induces apoptosis and inhibits the proliferation, migration, and invasion of the osteosarcoma MG-63 cell line in vitro. Tanshinone IIA (Tan IIA) is an active ingredient extracted from the widely used Danshen root (Salvia miltiorrhiza Bunge), a traditional Chinese medicine. Recent studies have indicated that Tan IIA may play important roles in anticancer treatment. However, its effects on the most common primary malignant bone tumor, osteosarcoma (OS), are unknown. Here, we report that Tan IIA may be an efficacious anti-OS drug as it could induce cell apoptosis and inhibit proliferation, migration, and invasion in vitro. Furthermore, we detected possible molecular mechanisms for Tan IIA activity by examining the levels of Bcl-2, Bax expression, and caspase-3, caspase-8, and caspase-9 activities that regulate apoptosis, matrix metalloproteinase (MMP)-2, and MMP-9 involved in regulating migration and invasion. In this study, we find that Tan IIA inhibits proliferation and induces apoptosis in the human OS cell line MG-63 in a time-dependent and dose-dependent manner. In addition, Tan IIA displays inhibitory activity on OS cell migration and invasion. Mechanistic studies have shown that Tan IIA activity is mediated by caspase activation. Tan IIA was also shown to reduce antiapoptotic Bcl-2, MMP-2, and MMP-9 levels, whereas it increased proapoptotic Bax levels. These data suggest that Tan IIA may be a novel, efficient candidate agent for OS treatment.