Serologic biomarker to predict clinical outcome is needed for immune checkpoint inhibitors (ICIs). We evaluated soluble intercellular adhesion molecules-1 (sICAM-1) as a predictor of response to ICIs treatment. Ninety-five patients with cancer treated with ICI were studied. The serum sICAM-1 levels of baseline, post two cycle therapy and end of therapy (EOT) were measured by enzyme-linked immunoassay. We randomly assigned the patients into the primary cohort (n = 47) and validation cohort (n = 48). Serum sICAM-1 post two cycle (277.7 ± 181.6 ng/mL) and EOT (403.9 ± 218.9 ng/mL) were significantly elevated compared to baseline (244.8 ± 153.8 ng/mL, p = 0.008 and p = 0.004, respectively). Early changes of sICAM-1 (ΔsICAM-1), deemed as sICAM-1 after two cycles minus baseline, were assessed. Following ICI treatments, responders had significantly lower ΔsICAM-1 compared with nonresponders in the primary cohort (p = 0.040) and the validation cohort (p = 0.026). High ΔsICAM-1 was strongly associated with inferior progression-free survival (PFS; (primary cohort: p = 0.001 and validation cohort: p = 0.002) and overall survival (OS; (primary cohort: p < 0.001 and validation cohort: p = 0.007). The ΔsICAM-1 remained independently associated with worse PFS and OS in the primary cohort and the validation cohort. Subgroup analysis indicated patients whose sICAM-1 significantly elevated had shorter PFS and OS in both anti-PD-1 and anti-PD-L1 treatment groups. Early change of serum sICAM-1 could be used to monitor and predict clinical benefit of ICI therapy in patients with solid cancer.