Abstract Background Talazoparib, a poly(ADP-ribose) polymerase enzyme inhibitor, is approved for the treatment of patients with germline BRCA1/2 (gBRCA1/2)-mutated HER2-negative advanced breast cancer. This two-part study, a recently published dose-escalation part followed by the dose-expansion part reported here, evaluated the efficacy and safety of talazoparib in Japanese patients with gBRCA1/2-mutated advanced breast cancer. Methods In this open-label, multicenter phase 1 study (NCT03343054), the primary endpoint of the dose-expansion part was confirmed objective response rate (ORR), determined by investigator assessment (RECIST 1.1). Secondary endpoints included progression-free survival (PFS), overall survival (OS), safety, and pharmacokinetics. Patients received the recommended phase 2 dose (1 mg/day; 0.75 mg/day moderate renal impairment). Results Nineteen Japanese patients with gBRCA1/2-mutated locally advanced or metastatic breast cancer were enrolled. Confirmed ORR was 57.9% (11/19; 90% confidence interval [CI] 36.8–77.0). Stable disease was observed in 36.8% (7/19) of patients. Per investigator assessment, median PFS was 7.2 months (95% CI 4.1–not estimable) and 12-month OS rate was 84.7% (90% CI 57.5–95.1). Median OS was not reached; 17/19 patients were alive and censored at 12 months. All patients experienced treatment-related adverse events (AEs); the majority were hematologic. The most common treatment-related AE was anemia (68.4%; [13/19]). Grade 3/4 treatment-related AEs were observed in 52.6% (10/19) of patients. During the safety period, there were no grade 5 treatment-emergent AEs, treatment-related serious AEs, or deaths. Conclusions In Japanese patients with gBRCA mutations and locally advanced or metastatic breast cancer, talazoparib monotherapy was generally well tolerated and resulted in clinically meaningful ORRs. Clinicaltrials.gov identifier NCT03343054.