Background: REG is a multikinase inhibitor with immunomodulatory activity and PEMBRO is an anti-PD-1 monoclonal antibody. Both are approved as monotherapy for patients (pts) with HCC previously treated with sorafenib. Based on their potential synergistic effects, we conducted a phase 1b study of REG plus PEMBRO for first-line treatment of advanced HCC. Methods: This is an ongoing, open-label, dose-escalation study in pts with advanced HCC who had no prior systemic therapy. In the first cohort, pts received REG 120 mg/day PO for 3 weeks on/1 week off plus PEMBRO 200 mg IV q 3 weeks. In later cohorts, the REG dose could be escalated (160 mg) or reduced (80 mg) based on the modified toxicity probability interval design; the PEMBRO dose is fixed. The primary objective is safety and tolerability. Secondary objectives are to define the maximum tolerated dose (MTD) and recommended phase 2 dose, and to assess antitumor activity. Results: As of August 23, 2019, 29 pts have been treated at the REG 120 mg level. Median age is 65 years (range 32–81); 41%/55% of pts are BCLC stage B/C; 100% are Child–Pugh A; ECOG status 0/1 is 72%/28%. Dose-limiting toxicities occurred in 4/18 evaluable pts: grade (Gr) 3 increased ALT/AST with Gr 2 increased bilirubin (n = 2); Gr 3 rash (n = 2). The MTD of REG in the combination was 120 mg. Most common Gr 3 or 4 treatment-emergent adverse events (TEAEs) are shown (n = 29). There were no Gr 5 TEAEs. 59%/31% of pts had REG/PEMRO-related Gr 3 or 4 TEAEs. Dose modifications (reductions or interruptions) of REG/PEMBRO for drug-related TEAEs occurred in 59%/31% of pts. Of 23 evaluable pts, 7 (30%) had a partial response (PR) and 14 (61%) had stable disease (RECIST v1.1); 1 additional pt had PR by mRECIST. Conclusions: The combination of REG plus PEMBRO for first-line treatment of advanced HCC showed no unexpected safety signals and encouraging antitumor activity. Accrual is continuing at REG 120 mg dose and an expansion cohort evaluating REG 80 mg plus PEMBRO is planned. Clinical trial information: NCT03347292