Background: Combining immune checkpoint inhibitors with chemotherapy has become a promising therapeutic strategy in metastatic breast cancer. Eribulin is a potent microtubule inhibitor and modulates the immune microenvironment of tumor cells. Therefore, combining eribulin to nivolumab may synergize antitumor efficacy in metastatic breast cancer. Methods: Adult patients with histologically confirmed recurrent/metastatic HER2- breast cancer were enrolled prospectively from 10 academic hospitals in Korea (ClinicalTrials.gov Identifier: NCT04061863). Key eligibility criteria included prior treatment with taxanes and/or anthracyclines, ≥1 measurable disease, and ≤2 prior cytotoxic chemotherapies in the metastatic setting. Patients received nivolumab 200 mg intravenously (IV) on day 1 plus eribulin 1.4 mg/m² IV on day 1 and 8 of every 3 weeks until disease progression or intolerable toxicity. The dose level was determined from safety profile of three patients in run-in phase. The primary endpoint was investigator-assessed progression-free survival (PFS) rate at 6 months. Secondary endpoints included investigator-assessed objective response rate (ORR) per RECIST v1.1, disease control rate (DCR), overall survival (OS), and toxicity profile of the combination treatment. The association between PD-L1 expression by SP142 Ab and efficacy was analyzed. Results: From August 2019 to June 2021, 90 patients (HR+HER2- 45 pts/TNBC 45 pts), with a median age of 51 (range 31–71), were enrolled in the study. With a median study follow-up time of 16.3 months, 68 (75.6%) patients experienced progressive disease. PFS rate at 6-months was 49.6% and 24.1% in patients with HR+HER2- and TNBC group, respectively. Median PFS was 5.6 months (95% CI: 4.3-6.8) and 3.0 months (95% CI: 1.3-4.7) for HR+HER2- and TNBC group, respectively. ORRs were 53.3% (CR:0, PR: 24) for HR+HER2- and 21.8% (CR1, PR: 12) for TNBC. Patients with PD-L1+ tumors (PD-L1 expression ≥ 1% on TC or IC) had similar ORR compared to PD-L1- tumors (ORR 50% vs. 53.8% in HR+HER2-, 30.8% vs. 29.0% in TNBC). The most common grade 3/4 adverse event was neutropenia (15/90, 16.7%), and the most common immune-related adverse events were grade 1/2 hypothyroidism (19/90, 21.1%) and grade 1/2 pruritus (16/90, 17.8%). Five patients had discontinued study treatment due to immune-related adverse events (3 pneumonitis, 1 hepatitis, 1 skin rash). Conclusions: In this parallel phase II clinical trial, the addition of nivolumab to eribulin showed promising efficacy and tolerable safety profile in previously treated HER2- MBC. Further survival and exploratory analyses to find predictive markers will be followed. Clinical trial information: NCT04061863.