Response rates to immune checkpoint blockade (ICB) in ovarian cancer are low. In preclinical studies, poly(ADP-ribose) polymerase inhibition (PARPi) synergized with CTLA4 ICB to achieve long-term survival in Brca1-deficient ovarian cancer models. Here we evaluated the safety and efficacy of combined treatment with the PARPi olaparib and the CTLA4 ICB tremelimumab for recurrent epithelial ovarian cancer in a multi-site single-arm phase I/II study enrolling 50 women with germline BRCA1 or BRCA2 mutations (NCT02571725). In phase I, no new toxicities were identified (n = 3). In the combined phase I and II safety cohort (n = 49), 57% of subjects experienced grade 3 or 4 non-hematologic toxicity, with tremelimumab discontinuation in 34.7%. The primary phase II endpoint, overall response rate was 32.7% (95% CI 19.5-45.8%), and disease control rate was 47.7% (95% CI 33.0-62.5%). Secondary outcomes were progression-free survival (PFS, 3.0 months, 95% CI 2.7-5.9), and overall survival (24.3 months, CI 14.9-30.4). In exploratory post-hoc biomarker analyses, low VSTM5 or IFIT1B expression was associated with treatment benefit and improved PFS (p 12 months in 6 subjects among this heavily pre-treated cohort highlight the need for biomarkers to select regimens for patients with recurrent ovarian cancer.