PURPOSE To evaluate the clinical benefit of nivolumab with or without ipilimumab in combination with stereotactic body radiotherapy (SBRT) in patients with refractory metastatic pancreatic cancer (mPC). METHODS Between November 2016 and December 2019, patients with refractory mPC were randomly assigned 1:1 to SBRT of 15 Gy with nivolumab or nivolumab/ipilimumab stratified by performance status (ClinicalTrials.gov identifier: NCT02866383 ). The primary end point was the clinical benefit rate (CBR), defined as the percentage of patients with complete or partial response (PR) or stable disease, according to RECIST 1.1. Simon's 2-stage phase II optimal design was used independently for both arms, with CBR determining expansion to the second stage. Secondary end points included safety, response rate, duration of response, progression-free survival, and overall survival. Exploratory analyses included biomarkers related to the benefits. RESULTS Eighty-four patients (41 SBRT/nivolumab and 43 SBRT/nivolumab/ipilimumab) received at least one dose of study treatment. CBR was 17.1% (8.0 to 30.6) for patients receiving SBRT/nivolumab and 37.2% (24.0 to 52.1) for SBRT/nivolumab/ipilimumab. PR was observed in one patient receiving SBRT/nivolumab and lasted for 4.6 months. Six patients receiving SBRT/nivolumab/ipilimumab achieved a PR with a median duration of response of 5.4 months (4.2 to not reached). Grade 3 or higher treatment-related adverse events occurred in 10 (24.4%) and 13 (30.2%) patients in the SBRT/nivolumab and SBRT/nivolumab/ipilimumab groups, respectively. Programmed cell death ligand-1 expression by tumor proportion score or combined positivity score of ≥ 1% was not associated with clinical benefits. On-treatment decreased serum interleukin-6, interleukin-8, and C-reactive protein levels were associated with better overall survival. CONCLUSION Clinically meaningful antitumor activity and favorable safety profiles were demonstrated after treatment with SBRT/nivolumab/ipilimumab in patients with refractory mPC. However, the contribution from SBRT is unknown. Further studies are warranted.