Background: Predicting the roughly 50% of melanoma patients that will respond to immunotherapy is challenging. We tested if splenic volume could be a predictive biomarker. Methods: Splenic volume was measured by a semiautomated commercial software tool in pre- and post-treatment PET/CT, CT or MRI in 50 melanoma patients treated with immune checkpoint inhibitors. Results: Subjects with smaller spleens had better progression-free survival (median not achieved after 30.6 months of follow-up vs median 11.2 months; p = 0.0213) than their counterparts. A cut-off of <244 cm³ yielded a sensitivity of 83% and specificity of 54% to identify responders. Conclusion: Measuring splenic volume on imaging scans is feasible. Smaller pretreatment spleen volume is associated with better responses to immune checkpoint inhibitors.