BACKGROUND: Immune checkpoint inhibition (ICI) significantly improves the survival of metastatic melanoma patients; however, a substantial proportion of patients does not respond to these breakthrough therapies. METHODS: To improve our understanding of this response variability, we developed high-plex panels for protein imaging of a discovery cohort and validation with RNAseq analyses to examine myeloid and T-cell subsets in pre-anti-PD-1-treatment samples of 14 metastatic melanoma patients (7 responders and 7 non-responders). RESULTS: We demonstrate that a higher abundance of circulating monocyte-derived macrophages (MDMs) and cytotoxic T-cell subsets in the tumor microenvironment (TME) at baseline distinguishes metastatic melanoma patients with a favorable response to anti-PD1 treatment from non-responders, who featured co-localization of suppressive macrophages (M2) and T-cells. Additionally, MDMs expressed high levels of immune checkpoints, and MDM infiltration into the TME was linked to both ICI response and survival. CONCLUSION: These findings highlight the potential of MDM infiltration as a predictive biomarker for ICI response in metastatic melanoma.