Background: Some studies have found that the application of traditional Chinese medicine in the treatment of lung cancer has achieved satisfying results. Polyphyllin Ⅲ (PP Ⅲ) is a natural steroid saponin from P. polyphylla var. yunnanensis, and its analogs have played a wide role in anticancer research. This study aimed to investigate the effect of PP Ⅲ on the development of lung cancer and its molecular mechanism. Methods: A549 and NCI-H1299 cell lines were treated with PP Ⅲ in gradient concentration to detect the IC50 of the cells, and the optimal concentration was selected for subsequent experiments. The effects of PP III treatment on lung cancer were investigated in vitro and in vivo. Results: In vitro experiments, it was found that the proliferation, invasion, migration, and colony formation ability of cancer cells were significantly reduced after PP III treatment, while accompanied by a large number of cell apoptosis. Further detection showed that N-cadherin was significantly decreased, E-cadherin was increased, and Snail and Twist were decreased in A549 cells and NCI- H1299 cells, respectively. In addition, GSK-3β expression was increased, while β-catenin expression was reduced with PP III treatment. In the mouse model, it was demonstrated that the volume of transplanted tumors was significantly reduced after PP Ⅲ treatment. Conclusions: PP Ⅲ has the capacity to inhibit the progression of lung cancer and regulate epithelial-mesenchymal transition through the GSK-3β/β-catenin pathway to suppress the malignant behavior of cancer cells. The application of PP Ⅲ is expected to be an effective method for the treatment of lung cancer