Background Despite the remarkable clinical advance of immune checkpoint inhibitors (ICIs) in the treatment of lung cancer, there are limited studies focused on evaluating efficacy of ICIs for patients with human epidermal growth factor receptor 2 (HER2)-mutant lung adenocarcinoma. Methods We conducted a multicenter retrospective study of patients with HER2-mutant lung adenocarcinoma who received ICIs therapy at Shanghai Pulmonary Hospital, Shanghai Chest Hospital and the First Affiliated Hospital of Wenzhou Medical University between 2016 and 2021. Response was defined with reference to the Response Evaluation Criteria in Solid Tumors (RECIST), version 1.1. Results Among the 26 patients enrolled in our study, the overall objective response rate (ORR) was 38.5%, disease control rate (DCR) was 84.6% and median progression-free survival (PFS) was 7.4 months. Majority of patients were treated with immunochemotherapy combination regimens (16/26, 61.5%), with a median PFS of 8.4 months. Among the 9 patients receiving ICIs-based therapy as first-line treatment, 5 patients had partial response (PR) and 4 patients had stable disease (SD), with a median PFS of 9.1 months. Of the entire cohort, 5 patients who received ICIs before epidermal growth factor receptor (EGFR)/HER2-targeting drugs achieved a median PFS of 8.4 months. Conclusion Our retrospective study provides clinical evidence that front line of ICIs-based therapy is also worth considering for the treatment to improve survival outcomes of patients with HER2-mutant lung adenocarcinoma.