Abstract Background Hypofractionated palliative radiotherapy for metastatic lung cancer patients is frequently used in order to ease pain, to increase bone stability, to treat local mass effects, or to prolong progression-free survival at critical sites. Recently introduced, immunotherapy for patients with non-squamous non-small cell lung carcinoma (NSCLC) has significantly improved outcome in this cohort. Preclinical and early clinical data suggest that the combination of photon radiation with programmed death-1 (PD-1) targeting immunotherapies may promote a strong and durable immune response against tumor manifestations both within and beyond radiation targets. Methods/design In the present prospective, two-group, non-randomized, open-label phase II trial, 130 patients with stage IV non-squamous NSCLC in 2nd-line or 3rd-line treatment will be included. 65 patients with a clinical indication for palliative radiotherapy to non-cerebral/non-pulmonary metastatic sites will receive 240 mg nivolumab followed by palliative radiotherapy with 5 × 4 Gray (Gy) = 20 Gy photon radiation, which will be initiated within 72 h after first nivolumab administration (Group A). 65 patients without an indication for radiotherapy will only receive nivolumab (Group B). Nivolumab will be further administered every two weeks in both groups and will be continued until progression and loss of clinical benefit or until occurrence of limiting toxicities. The primary endpoint will be the objective response rate (ORR) according to response evaluation criteria in solid tumors (RECIST) 1.1. Secondary endpoints will be progression-free survival (PFS) according to RECIST 1.1, overall survival, descriptive subgroup analyses according to PD-L1 expression, toxicity and quality of life. Since response patterns following immunotherapies differ from those after conventional cytostatic agents, both objective response rate and progression-free survival will additionally be assessed according to immune-related RECIST (irRECIST) criteria. Discussion The FORCE study will prospectively investigate response rates, progression-free and overall survival (OS), and toxicity of nivolumab with and without hypofractionated palliative radiotherapy in a group of 130 patients with metastatic non-small cell lung cancer (non-squamous histology) in 2nd-line or 3rd-line treatment. This trial will contribute prospective data to the repeatedly published observation that the combination of hypofractionated photon radiotherapy and medical immunotherapy is not only safe but will also promote antitumoral immune responses. Trial registration Clinicaltrials.gov identifier: NCT03044626 (Date of initial registration: 05 January 2017). Eudra-CT Number: 2015–005741-31 (Date of initial registration: 18 December 2015).