Background: Metastatic breast cancer (MBC) remains an incurable disease despite major therapeutic advances. Pseudomonas aeruginosa–mannose-sensitive hemagglutinin (PA-MSHA) has been established to have anti-proliferative effects against breast cancer cells in preclinical experiments, and is indicated for treatment of cancer in China. We performed a phase II trial combining PA-MSHA with capecitabine in patients with heavily pretreated MBC. Methods: Eligibility criteria included human epidermal growth factor receptor 2–negative MBC, prior therapy with anthracyclines and taxanes, at least one prior chemotherapy regimen for metastatic disease or early relapse after a taxane plus anthracycline adjuvant regimen, and adequate organ function and performance status. PA-MSHA 1 mg was administered subcutaneously every other day and capecitabine 2000 mg/m²orally twice a day for 2 weeks on, 1 week off. The primary end point was progression-free survival. The trial was registered at www.ClinicalTrials.gov (ID NCT01380808). Results: A total of 97 patients were enrolled. Median progression-free survival (PFS) was 4.0 months [95 % confidence interval (CI) 3.0–4.9), which was not significantly different from that in historical controls. However, median PFS was significantly longer (8.2 months; 95 % CI 6.7–9.7) in 24 patients with moderate immune-related adverse events (irAEs) such as fever or skin induration at the injection site than in those with no or mild irAEs (3.1 months, 95 % CI 2.5–3.6; p = 0.003). Overall survival was also improved in these patients (25.4 vs. 16.4 months; p = 0.044). PA-MSHA has a good safety profile, with only 6 patients (6.2 %) discontinuing treatment. PA-MSHA did not increase capecitabine-related toxicities such as hand-foot syndrome, nausea, and vomiting. Conclusions: Adding PA-MSHA to capecitabine has a good safety profile in patients with heavily pre-treated MBC, although benefit from this regimen might occur only in patients with moderate PA-MSHA–related adverse events. Clinical trial information: NCT01380808.