Background The prognosis of hepatocellular carcinoma (HCC) with major portal vein tumor thrombosis (PVTT) is dismal after standard treatment with sorafenib. Hepatic arterial infusion chemotherapy (HAIC) has been suggested for patients with HCC and major PVTT. Purpose To compare the efficacy and safety of sorafenib plus 3cir-OFF HAIC versus sorafenib alone for advanced HCC with major PVTT. Materials and Methods This phase II trial recruited systemic treatment-naïve patients with HCC and major PVTT (portal vein invasion grade Vp3 [first branch] and Vp4 [main trunk]) between June 2017 and November 2019. Patients were randomly assigned (1:1 ratio) to receive sorafenib (400 mg twice daily) plus 3cir-OFF HAIC (35 mg/m2 oxaliplatin [hours 0-2] followed by 600 mg/m2 5-fluorouracil [hours 2-24], days 1-3) with a standardized percutaneous port catheter system or sorafenib alone (400 mg twice daily) every 4 weeks. The primary end point was overall survival (OS). The secondary end points were objective response rate, progression-free survival (PFS), and safety. OS and PFS were assessed using the Kaplan-Meier method and log-rank test. Results The intent-to-treat population included 64 patients, with 32 in each group. The median OS was 16.3 months (95% CI: 0.0, 35.5) with sorafenib plus HAIC and 6.5 months (95% CI: 4.4, 8.6) with sorafenib alone (hazard ratio [HR] = 0.28; 95% CI: 0.15, 0.53; P < .001). A higher objective response rate (41% [n = 13] vs 3% [n = 1], P < .001) and a longer median PFS (9.0 months vs 2.5 months; HR = 0.26; 95% CI: 0.15, 0.47; P < .001) were observed in the sorafenib plus HAIC group. Grade 3 or 4 adverse events were more frequent in the sorafenib plus HAIC group, including diarrhea (n = 7 [22%] vs n = 5 [16%]), hand-foot syndrome (n = 6 [19%] vs n = 2 [6%]), and thrombocytopenia (n = 7 [22%] vs n = 0). Conclusion Sorafenib plus 3cir-OFF hepatic arterial infusion chemotherapy may be a promising treatment in patients with hepatocellular carcinoma and major portal vein tumor thrombosis because of the improved survival and an acceptable safety profile. Clinical trial registration no. NCT03009461 © RSNA, 2022 Online supplemental material is available for this article. See also the editorial by Chung in this issue.