Nivolumab, an anti PD-1 monoclonal antibody, has been approved for the treatment of previously treated advanced or metastatic non-small-cell lung cancer (NSCLC). The aim of this study was to develop and validate an ELISA method for the quantification of nivolumab in plasma from patients with NSCLC in order to perform future pharmacokinetic/pharmacodynamic (PK/PD) studies. A home-made ELISA was developed and validated according to the general recommendations for the immunoassays. Then, the ELISA method was applied to quantify plasma trough levels (Cmin) of nivolumab (3 mg/kg every two weeks) in 27 NSCLC patients at days 14, 28 and 42 after start of treatment. Blood samples were collected just before the infusion on days 0 (baseline), 14, 28 and 42 after start of treatment. The dynamic calibration range for nivolumab assay was 5–100 μg/mL. Within- and between-day imprecision for quality controls (5, 20 and 75 μg/mL) were less than 5 and 12%, respectively. The mean (± standard deviation) nivolumab Cmin was 17.3 ± 4.8 μg/mL (coefficient of variation, CV = 27.8%), 25.0 ± 9.7 μg/mL (CV = 38.8%) and 33.0 ± 12.9 μg/mL (CV = 39.1%) on days 14, 28 and 42, respectively. IgG (p = 0.002) and ALT (p = 0.041) were independently associated with plasma nivolumab Cmin at day 42. The present ELISA method for quantification of nivolumab in plasma from NSCLC patients is sensitive and accurate enough to be used for further PK/PD investigations.