Polyphyllins, a major component of Rhizoma paridis, have been extensively used in non-small cell lung cancer (NSCLC). The aim of the present study was to evaluate the effects of Paris saponin I (PSI) on a panel of gefitinib-resistant NSCLC cell lines and its inhibition of tumor growth in a nude mouse model. The MTT assay was used to assess growth inhibition. The cell cycle was analyzed using flow cytometry and apoptosis was assessed using Annexin V/propidium iodide staining. The morphology of the apoptotic cells was determined by transmission electron microscopy. The protein expression levels of B-cell lymphoma 2 (Bcl-2), Bcl-2-associated X protein (Bax) and caspase-3 were detected using western blot analysis. In addition, the glucose metabolism in tumor-bearing mice was evaluated using 18F-fludeoxyglucose (FDG) micro-positron emission tomography imaging. The PSI-induced growth inhibition rate was observed to significantly increase in a time- and dose-dependent manner. Furthermore, PSI induced significant G2/M-phase arrest and apoptosis. The expression levels of Bcl-2 decreased, while those of Bax and caspase-3 increased following PSI treatment. 18F-FDG-uptake in the PSI treatment groups was significantly decreased compared with that in the control group in vivo. In conclusion, PSI is a potent antitumor agent that acts by inhibiting the proliferation of gefitinib-resistant cells, and has potential as a candidate for a natural drug for gefitinib-resistant therapy. PSI-induced apoptosis, which occurred via multiple pathways, including G2/M-phase arrest and upregulation of the Bax/Bcl-2 ratio and caspase-3 expression, ultimately led to cell death and tumor inhibition.