Purpose We aimed to assess the prognostic and predictive significance of pretreatment Onodera's prognostic nutritional index (OPNI) in metastatic non-small cell lung cancer patients (NSCLC) treated with first-line chemotherapy. Materials and methods Patients with metastatic NSCLC who attended five different medical oncology clinics between December 2008 and January 2018 were retrospectively analyzed. The optimal cut-off point for OPNI was performed by a receiver operating characteristic (ROC) curve analysis. Patients were assigned to either the low OPNI group or high OPNI group. Results A total of 333 patients were included in the study. Significant differences between the low and high OPNI groups were found regarding the rates of response to chemotherapy, sex, and hemoglobin level (p < 0.05). The patients in high OPNI group had a longer overall survival (OS) (15.3 vs. 10.6 months, p < 0.001) and progression-free survival (PFS) (6.7 vs. 5.3 months, p < 0.001) compared to the patients in low OPNI group. A multivariate analysis using Cox regression model revealed that a high OPNI score was an independent prognostic factor of OS (HR = 1.535, p = 0.002) and PFS (HR = 1.336, p = 0.014), but failed to demonstrate a statistical significance of pretreatment OPNI scores in predicting treatment response (p = 0.56). Conclusions Pretreatment OPNI is an independent prognostic factor for OS and PFS in metastatic NSCLC patients treated with first-line chemotherapy. Thus, it may be used as easily calculated and low-cost prognostic tool in the routine clinical practice in this patient group.