Nutritional status has been associated with prognosis in several cancers. We investigated whether baseline prognostic nutritional index (PNI) and geriatric nutritional risk index (GNRI) as well as their variations during treatment predicted response and survival in extensive-stage small cell lung cancer (ES-SCLC) patients treated with atezolizumab plus chemotherapy. In this multicenter study, records of ES-SCLC patients who received first-line atezolizumab plus platinum-etoposide combination were reviewed retrospectively. Baseline PNI <45 and GNRI <98 were accepted as low. They were reassessed on day 1 of the third cycle to calculate changes from the baseline (ΔPNI and ΔGNRI). Regression models were used to determine predictive factors for response, progression-free and overall survival (PFS and OS). The study included 145 patients. High baseline PNI was independently associated with objective response (odds ratio=2.50, P=0.02). In patients with a low baseline PNI, median PFS was significantly shorter (6.1 vs. 8.7 mo, P=0.04) and it significantly predicted PFS (hazard ratio=1.52, P=0.03). Median OS was significantly shorter in patients with ΔPNI ≤-10% (11.1 vs. 14.9 mo, P=0.01), which independently predicted OS (hazard ratio=2.10, P=0.001). Baseline GNRI and ΔGNRI were not associated with efficacy. However, in patients 65 years of age or older, median PFS was significantly shorter in cases with a low baseline GNRI (7.1 vs. 10.7 mo, P=0.04). PNI and its variations as convenient and cost-effective markers can help predict response and prognosis in ES-SCLC patients receiving first-line atezolizumab plus chemotherapy. GNRI might emerge as an additional prognostic tool in patients 65 years of age or older.