Background: Breast cancer is the most common cancer among women, with up to 37% of patients (pts) harboring germline BRCA1/2 mutations (gBRCA1/2m) that appear to be sensitive to poly (ADP-ribose) polymerase proteins 1 and 2 (PARP1/2) inhibition. Pamiparib is an orally administered selective PARP1/2 inhibitor that has the potential to cross the blood-brain barrier. This study evaluated the efficacy and safety of pamiparib in pts with locally advanced/metastatic human epidermal growth factor receptor 2-negative (HER2-) breast cancer, with deleterious or suspected deleterious gBRCA1/2m, who received ≤ 2 prior lines of chemotherapy. Methods: In this open-label, phase 2, multi-center study in China (NCT03575065), pts with locally advanced/metastatic HER2- breast cancer with deleterious or suspected deleterious gBRCA1/2m triple negative breast cancer (TNBC cohort) or hormone receptor-positive (HR+)/HER2- breast cancer (HR+ cohort) were enrolled. Pts received pamiparib 60 mg orally twice daily in 28-day cycles. The primary endpoint was objective response rate (ORR; RECIST v1.1) by independent review committee (IRC). Secondary endpoints included duration of response (DOR) and progression free survival (PFS) by IRC, overall survival (OS), safety and tolerability. Results: 88 pts were enrolled (median age 45.5 years), 76 pts (TNBC cohort n = 55; HR+ cohort n = 21) had measurable disease at baseline per IRC. 60 pts (68.2%) received 1 or 2 prior lines of chemotherapy; 42 pts (47.7%) were treated with platinum previously. Median follow-up was 13.77 months (TNBC cohort, 10.87 months; HR+ cohort, 18.45 months). In the TNBC cohort: confirmed ORR was 38.2% (95% CI: 25.4–52.3); median DOR (mDOR) was 6.97 months (95% CI: 3.94–not estimable[NE]); median PFS (mPFS) was 5.49 months (95% CI: 3.65–7.33); median OS (mOS) was 17.08 months (95% CI:13.70–NE). In the HR+ cohort: confirmed ORR was 61.9% (95% CI: 38.4–81.9); mDOR was 7.49 months (95% CI: 5.55–14.75); mPFS was 9.20 months (95% CI: 7.39–11.93); mOS was not reached (NR; 95% CI 18.10–NE). ≥ Grade 3 treatment emergent adverse events (TEAEs) occurred in 54 pts (61.4%); anemia was the most common TEAE, occurring in 77 pts (87.5%). Dose reduction due to TEAEs occurred for 57 pts (64.8%); discontinuations due to TEAEs occurred for 2 pts (2.3%). Conclusions: Pamiparib showed a promising response in pts with locally advanced/metastatic HER2- breast cancer with a gBRCA1/2m. The safety profile of pamiparib was considered acceptable and was generally consistent with therapies in the same class. Clinical trial information: NCT03575065.