Objectives: Blockade of programmed cell death-1 (PD-1) and its ligand (PD-L1) has transformed the treatment of NSCLC. In a first-in-human, Phase 1, dose escalation and cohort expansion study, cemiplimab, a monoclonal antibody directed against PD-1, was evaluated for the treatment of patients with advanced solid tumors (NCT02383212). Here, we report results in patients with advanced NSCLC from the dose expansion cohort. Materials and methods: Immune-checkpoint inhibitor naive patients with advanced NSCLC (stage III/IV), irrespective of PD-L1 status, who had progressed after, or were refractory to first- or later-line therapy were enrolled and received cemiplimab 200 mg every 2 weeks intravenously for up to 48 weeks. Primary study objectives were to assess safety and tolerability, and to evaluate clinical activity of cemiplimab. Results: Twenty patients with NSCLC were enrolled. Median age was 64.0 years (range: 50–82); 65.0 % were male; 80.0 % had an ECOG performance status of 1; 60.0 % had a histology of adenocarcinoma. Median number of prior lines of systemic therapy was 2 (range: 1–4). Median duration of follow-up was 7.0 months (range: 1.0–18.2). All patients experienced ≥1 treatment-emergent adverse event (TEAE) of any grade. Most common TEAEs were arthralgia, asthenia, cough, and dyspnea (each 4/20; 20.0 %). Grade ≥3 TEAEs occurred in 60.0 % (12/20) of patients. Of patients with measurable disease per independent central review (ICR), five had partial response (PR), four had stable disease (SD) and 10 had progressive disease. Objective response rate (ORR; complete response + PR) was 25.0 % (95 % CI: 8.7–49.1 %). Duration of response exceeded 8 months in four of the five responding patients at the time of data cut-off (April 30, 2019). The disease control rate per ICR (ORR + SD) was 50.0 % (95 % CI: 27.2–72.8 %). Conclusion: Cemiplimab showed an acceptable safety profile and demonstrated antitumor activity in pretreated patients with NSCLC.