Introduction <p>There is a need for biomarkers to predict response and survival to immune checkpoint inhibitors (ICIs) in patients with advanced non-small cell lung cancer (NSCLC). Soluble PD-L1 (sPD-L1) has shown biomarker potential. The objective of this study was to evaluate sPD-L1 in patients with advanced NSCLC treated with first-line ICIs.</p> Methods <p>We constructed three prospective cohorts of patients with advanced NSCLC treated with first-line chemotherapy (CT), (Cohort #1), ICIs, or CT-ICIs (Cohort #2 and #3). Plasma was collected at baseline and at first tumour evaluation. sPD-L1 levels were measured by ELISA and compared to response and survival metrics.</p> Results <p>Patients were mostly male smokers with adenocarcinomas. Baseline sPD-L1 was lower in responders versus&#xa0;(vs) non-responders in Cohort #2 (<i>p</i> = 0.0233). Patients with low baseline sPD-L1 had longer OS in Cohorts #2 and #3: median OS 18.0&#xa0;months vs 4.0&#xa0;months, (<i>p</i> = 0.0277) and not reached (NR) vs 13.0&#xa0;months (<i>p</i> = 0.0360). First tumour evaluation sPD-L1 was lower in responders in Cohorts #1 (<i>p</i> = 0.0138) and #2 (<i>p</i> = 0.0009). Patients with low sPD-L1 at first tumour evaluation had longer OS in Cohort #2: 45.0&#xa0;months vs 12.5 (<i>p</i> = 0.0041). Patients with stable/decreasing sPD-L1 had longer OS throughout the Cohorts: median OS of 15.5 vs 6.0&#xa0;months, 45.0 vs 14.0&#xa0;months and not reached (NR) vs 17.0&#xa0;months in Cohorts #1, #2 and #3. In vitro studies confirmed that cancer and immune cells secreted sPD-L1 and that NSLC patient plasma has the capacity to inhibit lymphocyte proliferation.</p> Conclusion <p>sPD-L1 has prominent biomarker potential in advanced NSCLC treated with first-line ICIs.</p>