Abstract Purpose To evaluate the efficacy and safety of pamiparib in patients with locally advanced or metastatic human epidermal growth factor receptor 2-negative (HER2−) breast cancer, with deleterious or suspected deleterious germline BRCA1/2 mutations (gBRCA1/2 m). Methods In this open-label, phase II, multicenter study in China (NCT03575065), patients with triple-negative breast cancer (TNBC cohort) or hormone receptor-positive (HR+)/HER2− breast cancer (HR+/HER2− cohort) and ≤ 2 prior lines of chemotherapy received pamiparib 60 mg orally twice daily in 28-day, continuous cycles. The primary endpoint was objective response rate (ORR; RECIST v1.1) by independent review committee. Results In total, 88 patients were enrolled (TNBC cohort: 62; HR+/HER2− cohort: 26). Median age was 45.5 (range: 27–67) years, and 60 patients (68.2%) had received 1 or 2 prior lines of chemotherapy; 42 patients (47.7%) had previously received platinum chemotherapy. In the TNBC cohort, ORR was 38.2% (95% confidence interval [CI] 25.4–52.3) and median duration of response (DoR) was 7.0 months (95% CI 3.9–not estimable). In the HR+/HER2− cohort, ORR was 61.9% (95% CI 38.4–81.9) and median DoR was 7.5 months (95% CI 5.6–14.8). The most common treatment-emergent adverse events (TEAEs), treatment-related TEAEs, and ≥ Grade 3 TEAEs were hematologic (including anemia, decreased neutrophil count, and decreased white blood cell count). Overall, 64.8% of patients had TEAEs leading to dose reduction and 2.3% had TEAEs leading to treatment discontinuation. Conclusion Pamiparib showed encouraging efficacy and an acceptable safety profile in patients with locally advanced and metastatic HER2− breast cancer with gBRCA1/2 m. Trial registration ClinicalTrials.gov, NCT03575065; July 2, 2018.