BACKGROUND & AIMS: Validated surrogate endpoints for overall survival (OS) are important for expediting the clinical study and drug-development processes. Herein, we aimed to validate objective response as an independent predictor of OS in individuals with unresectable hepatocellular carcinoma (HCC) receiving systemic anti-angiogenic therapy. METHODS: We investigated the association between objective response (investigator-assessed mRECIST, independent radiologic review [IRR] mRECIST and RECIST v1.1) and OS in REFLECT, a phase III study of lenvatinib vs. sorafenib. We conducted landmark analyses (Simon-Makuch) of OS by objective response at 2, 4, and 6 months after randomization. RESULTS: Median OS was 21.6 months (95% CI 18.6-24.5) for responders (investigator-assessed mRECIST) vs. 11.9 months (95% CI 10.7-12.8) for non-responders (hazard ratio [HR] 0.61; 95% CI 0.49-0.76; p <0.001). Objective response by IRR per mRECIST and RECIST v1.1 supported the association with OS (HR 0.61; 95% CI 0.51-0.72; p <0.001 and HR 0.50; 95% CI 0.39-0.65; p <0.001, respectively). OS was significantly prolonged for responders vs. non-responders (investigator-assessed mRECIST) at the 2-month (HR 0.61; 95% CI 0.49-0.76; p <0.001), 4-month (HR 0.63; 95% CI 0.51-0.80; p <0.001), and 6-month (HR 0.68; 95% CI 0.54-0.86; p <0.001) landmarks. Results were similar when assessed by IRR, with both mRECIST and RECIST v1.1. An exploratory multivariate Cox regression analysis identified objective response by investigator-assessed mRECIST (HR 0.55; 95% CI 0.44-0.68; p <0.0001) and IRR-assessed RECIST v1.1 (HR 0.49; 95% CI, 0.38-0.64; p <0.0001) as independent predictors of OS in individuals with unresectable HCC. CONCLUSIONS: Objective response was an independent predictor of OS in individuals with unresectable HCC in REFLECT; additional studies are needed to confirm surrogacy. Participants achieving a complete or partial response by mRECIST or RECIST v1.1 had significantly longer survival vs. those with stable/progressive/non-evaluable disease. GOV NUMBER: NCT01761266. IMPACT AND IMPLICATIONS: This analysis of data taken from a completed clinical trial (REFLECT) looked for any link between objective response and overall survival time in individuals with unresectable HCC receiving anti-angiogenic treatments. Significantly longer median overall survival was found for responders (21.6 months) vs. non-responders (11.9 months). Overall survival was also significantly longer for responders vs. non-responders (based on objective response status at 2, 4, and 6 months) in the landmark analysis. Our results indicate that objective response is an independent predictor of overall survival in this setting, confirming its validity as a rapid marker of efficacy that can be applied in phase II trials; however, further validation is required to determine is validity for other systemic treatments (e.g. immunotherapies), or as a surrogate of overall survival.