Introduction: Previous studies have suggested that the efficacy of eribulin is influenced by the activity of antitumor immunity of patients. Absolute lymphocyte count (ALC) and the neutrophil/lymphocyte ratio (NLR) are easily available parameters associated with the immunological status of patients. Objective: Here we tried to classify patients’ immunological status by using the scatter plot of ALC and NLR, and investigated its utility for predicting survival among patients with metastatic breast cancer receiving eribulin. Methods: The medical records of 125 patients who received eribulin for metastatic breast cancer at our hospital between July 2011 and April 2019 were retrospectively reviewed. Uni- and multivariate analyses were performed to determine the association between baseline ALC/NLR and progression-free survival (PFS)/overall survival (OS). The cutoff values for ALC and NLR were determined using scatter plot analysis. Results: The entire cohort was classified into immunologically favorable (ALC ≥1,500/µL, 30 patients), intermediate (ALC <1,500/µL, NLR <5.0, 76 patients), and unfavorable (NLR ≥5.0, 19 patients) groups. Univariate analysis showed significant differences in PFS and OS between the groups, whereas multivariate analysis revealed that ALC ≥1,500/µL and NLR ≥5.0 were independent predictors of PFS, with adjusted hazard ratios (95% CI) of 0.57 (0.33–0.99) and 1.78 (1.00–3.15), respectively. NLR ≥5.0 was also associated with worse OS (adjusted hazard ratio: 0.55; 95% CI 0.35–0.88; p = 0.013). Conclusions: Among patients with metastatic breast cancer receiving eribulin, survival outcomes were well stratified according to baseline peripheral blood ALC and NLR. Accordingly, high ALC and NLR can be used as predictive markers for longer disease control and worse survival, respectively.