Background: Pembrolizumab and other immune check-point inhibitors became the mainstay of treatment in ample malignancies, including non-small cell lung cancer (NSCLC). Diabetes mellitus (DM) is a highly prevalent chronic metabolic disease. Though DM causes immune dysfunction, the effect of DM on immunotherapy efficacy has not been addressed. Methods: Medical records of consecutive NSCLC patients treated with first-line pembrolizumab alone or combined with chemotherapy at Tel Aviv Medical Center from January 2017 to July 2021 were reviewed. We excluded patients who received a single cycle or were lost to follow-up. Results: Of 234 patients reviewed, 203 were included in the analysis. The median age was 69 years, 128 were men (63%), 152 had adenocarcinoma (75%), 105 received monotherapy pembrolizumab (52%), and 51 patients had DM (25%). Diabetic patients were older (73 vs. 67, p < 0.001) and had a higher mean body index (27 vs. 24, P < 0.001). Median progression free survival (PFS) and overall survival (OS) were significantly shorter in diabetic compared to non-diabetic patients (5.9 vs. 7.1 months, respectively, p = 0.004, and OS 12 vs. 21 months, respectively, p = 0.006). The difference in OS was more pronounced for patients receiving pembrolizumab alone (12 vs. 27 months, p = 0.03), than for those receiving pembrolizumab together with chemotherapy (14.3 vs. 19.4 months, p = 0.06). This suggests that DM is mainly determinantal to anti-tumor immunity and not to the cytotoxic effect of chemotherapy. Multivariate analysis indicated DM as an independent risk factor for inferior PFS (HR 1.7, 95% CI 1.11-2.5, p = 0.014) and OS (HR 1.7, 95% CI 1.09-2.76, p = 0.02). Conclusions: Here we show a potential deleterious effect of DM on the efficacy of pembrolizumab in metastatic NSCLC patients. If further validated, the administration of single agent pembrolizumab in this setting should be reconsidered.