BACKGROUND: Immune checkpoint inhibitors (ICIs) have revolutionized the treatment of advanced melanoma, but racial disparities in melanoma outcomes continue. These inequities are not fully explained by individual factors. OBJECTIVE: To investigate the associations of neighborhood factors with the use of ICIs in metastatic melanoma. METHODS: We conducted a retrospective cohort study of commercially insured US adults with metastatic melanoma diagnosed between January 2011 and December 2020. We examined the associations between the county-level percentage of population from racial and ethnic minority groups and the time from metastatic melanoma diagnosis to initiating ICIs using Cox proportional hazards models adjusting for patient characteristics. RESULTS: We identified 4,052 patients with metastatic melanoma, of which 49% used ICIs. We found that the adoption of ICIs in a county declined with increasing minority quintile (quintile 1: 52.4%, quintile 2: 50.4%, quintile 3: 50.1%, quintile 4: 45.8%, and quintile 5: 44.7%). The delay in ICI initiation also went up as the percentage of minorities in a county increased (log-rank test P = 0.03). Compared with the lowest quintile, the adjusted hazard ratio of ICI initiation of the second, third, fourth, and highest minority quintile was 0.94 (95% CI = 0.81-1.08), 0.88 (95% CI = 0.76-1.02), 0.81 (95% CI = 0.68-0.97), and 0.77 (95% CI = 0.66-0.91), respectively. Secondary analysis revealed that the slower initiation was driven by the counties with the highest percentage of Hispanic population (hazard ratio = 0.74; 95% CI = 0.61-0.89) in both Cox models and sensitivity analyses. High-minority counties correlated with metro areas, higher poverty levels, and a greater number of medical oncologists. CONCLUSIONS: We found that patients with metastatic melanoma living in counties with higher proportion of minorities, particularly of Hispanic origin, are more likely to experience delays in ICI treatment. This study provides important population-level data on neighborhood-level disparity in medication use. More research is needed on the underlying provider- and system-level factors that directly contributed to the lower use of cancer medicines in high-minority areas, which can help inform the development of evidence-based medication use strategies that can improve health outcomes and equity.