Background <p>Nivolumab was first approved to treat patients with advanced non-small cell lung cancer (NSCLC) in 2015. Since then, NSCLC treatment has entered the immunotherapy era. However, studies on the effect of chemotherapy in the immunological era on elderly lung adenocarcinoma (LUAD) patients with brain metastases are lacking. This study aimed to analyze the effect of chemotherapy on the overall survival (OS) of elderly LUAD patients with brain metastases in the immunotherapy era.</p> Methods <p>This study included elderly patients (≥ 80 years) diagnosed with LUAD and brain metastases in the Surveillance, Epidemiology, and End Results (SEER) database from 2015 to 2021. Patients were divided into chemotherapy and non-chemotherapy groups. Propensity score matching (PSM) was utilized to reduce bias between two groups. Survival analyses were performed using Kaplan-Meier curves and Cox regression models.</p> Results <p>Multivariate analysis revealed that chemotherapy (HR 0.366; 95% CI 0.325–0.412; <i>P</i> &lt; 0.001) was an independent prognostic factor affecting OS for elderly LUAD patients with brain metastases in the era of immunotherapy. Survival analysis showed that the median OS was higher in the chemotherapy group than in the non-chemotherapy group (10 months vs. 2 months) before and after PSM. Subgroup analysis indicated that the risk of death was lower in the group that received chemotherapy compared to the group that did not receive chemotherapy, particularly in patients with T3-4, N2-3, bone metastases, liver metastases, and lung metastases.</p> Conclusions <p>Chemotherapy may still be indispensable for patients with advanced LUAD and brain metastases in the era of immunotherapy.</p>