Background: Currently, the landscape of treating advanced malignancies has undergone a transformative shift with the advent of immunotherapy employing immune checkpoint inhibitors (ICIs). Notably, several ICIs have emerged as promising therapeutic modalities for individuals with head and neck cancer (HNC). An emerging body of evidence implies a plausible link between body mass index (BMI) and the effectiveness of ICIs in the broader context of cancer patients. Nevertheless, the specific correlation within the subset of head and neck cancer patients undergoing immunotherapeutic interventions remains unclear and warrants meticulous investigation. Methods: PubMed, Web of Science, and Google Scholar databases were searched extensively for records published until January 2024. Full-text articles aligned with the research objective were included, while records published in English, case reports, reviews, editorials, and studies reporting immunotherapy combined with other cancer therapies were excluded. The data required for review and analysis was abstracted in Excel files by two independent reviewers. Additionally, statistical analyses were performed using the Review Manager software, and methodological quality was assessed using the Newcastle Ottawa scale. Results: Only six studies were eligible for review and analysis. A subgroup analysis of data from these studies showed that obese HNC patients on immunotherapy had significantly better overall survival (OS) rates than non-obese patients (HR: 0.51; 95% CI: 0.29 – 0.93; p=0.03). However, the progression-free survival (PFS) was statistically similar between obese and non-obese patients (HR: 0.72; 95% CI: 0.39 – 1.33; p=0.30). In addition, when BMI was stratified as either low or high, no significant difference was observed in the OS and PFS of HNC patients (HR: 0.99; 95% CI: 0.59 – 1.66; p=0.97 and HR: 0.93; 95% CI: 0.61 – 1.41; p=0.42, respectively). Similarly, the statistical analyses showed that overweight patients have similar OS and PFS as patients with normal BMI (HR: 0.53; 95% CI: 0.15 – 1.92; p=0.33 and HR: 0.55; 95% CI: 0.20 – 1.52; p = 0.25, respectively). In contrast, underweight patients demonstrated poor OS and PFS (HR: 2.56; 95% CI: 1.29 – 5.12; p=0.008 and HR: 2.76; 95% CI: 1.17 – 6.52; p=0.02, respectively). Conclusions: Obese HNC patients on immunotherapy tend to have improved OS than non-obese patients, while underweight patients have worse clinical prognoses than those with normal or above BMI.