Thrombosis is a well‐known cardiovascular disease (CVD) complication that has caused death in many patients with cancer. Oral bacteria have been reported to contribute to systemic diseases, including CVDs, and tumor metastasis. However, whether oral bacteria‐induced thrombosis induces tumor metastasis remains poorly understood. In this study, the cariogenic oral bacterium Streptococcus mutans was used to examine thrombosis in vitro and in vivo. Investigation of tumor metastasis to the lungs was undertaken by intravenous S. mutans implantation using a murine breast cancer metastasis model. The results indicated that platelet activation, aggregation, and coagulation were significantly altered in S. mutans‐stimulated endothelial cells (ECs), with elevated neutrophil migration, thereby inducing thrombosis formation. Streptococcus mutans stimulation significantly enhances platelet and tumor cell adhesion to the inflamed ECs. Furthermore, S. mutans‐induced pulmonary thrombosis promotes breast cancer cell metastasis to the lungs in vivo, which can be reduced by using aspirin, an antiplatelet drug. Our findings indicate that oral bacteria promote tumor metastasis through thrombosis formation. Oral health management is important to prevent CVDs, tumor metastasis, and their associated death. The cariogenic oral bacterium Streptococcus mutans promotes platelet and coagulation pathway activation in endothelial cells to induces thrombosis. Streptococcus mutans‐induced pulmonary thrombosis promotes breast cancer cell metastasis to the lungs. Thrombosis is a well‐known cardiovascular disease complication that has caused death in many patients with cancer. Our findings indicate that oral health management is important to prevent cardiovascular disease, tumor metastasis, and their associated death.