Lynch syndrome is caused by mutations in mismatch repair genes that lead to microsatellite instability (MSI). An increased number of mutation-associated neoantigens have been observed in patients with high-frequency MSI (MSI-H) cancer; in addition, membranous programmed death ligand-1 (PD-L1) tends to be expressed at higher levels in MSI-H cancers than in microsatellite-stable cancers. MSI-H cancer patients are therefore considered to be susceptible to immune checkpoint blockade. We herein report for the first time a case of lung adenocarcinoma with Lynch syndrome and the response to nivolumab.