Purpose <p>The value of adjuvant immunotherapy in patients with resectable stage III non-small cell lung cancer (NSCLC) after neoadjuvant chemoimmunotherapy remains unclear. This study aimed to evaluate the prognostic impact of additional adjuvant immunotherapy in patients with stage III NSCLC.</p> Methods <p>Patients with stage III NSCLC who received neoadjuvant chemoimmunotherapy followed by radical surgery, with or without adjuvant immunotherapy, were retrospectively enrolled across two hospitals. Event-free survival (EFS) and overall survival (OS) were assessed from the initiation of neoadjuvant treatment and were estimated by the Kaplan‒Meier method. One-to-one propensity score matching (PSM) and inverse probability of treatment weighting (IPTW) were used to minimize confounding.</p> Results <p>A total of 184 eligible patients were enrolled, of whom 105 (57.1%) received adjuvant immunotherapy and 79 (42.9%) did not. After 1:1 PSM, the addition of adjuvant immunotherapy did not significantly improve EFS (2-year EFS: 62.3% vs. 66.1%, <i>P</i> = 0.653) or OS (2-year OS: 92.7% vs. 89.6%, <i>P</i> = 0.196). Subgroup analyses, stratified by the pathological complete response (pCR) status, further confirmed that adjuvant immunotherapy did not significantly improve survival in either the pCR subgroup or the non-pCR subgroup. Similar results were obtained after IPTW. Exploratory analysis revealed that 3 cycles of neoadjuvant immunotherapy might be more beneficial than 2 (pCR: 40.8% vs. 30.6%, <i>P</i> = 0.292; 2-year EFS: 75.0% vs. 54.5%, <i>P</i> = 0.111) or 4 (pCR: 42.1% vs. 36.8%, <i>P</i> = 0.740; 2-year EFS: 63.2% vs. 51.5%, <i>P</i> = 0.343) cycles.</p> Conclusion <p>The addition of adjuvant immunotherapy to neoadjuvant chemoimmunotherapy may not be necessary in patients with resectable stage III NSCLC. Three cycles of neoadjuvant immunotherapy appear to be an appropriate treatment regimen for neoadjuvant chemoimmunotherapy.</p>