Background: This study compared the effectiveness of the combined administration of sorafenib, an immune checkpoint inhibitor, transcatheter arterial chemoembolization (TACE), and stereotactic body radiation therapy (SBRT) (SITS group) vs. sorafenib combined with TACE (ST group) in treating and downstaging advanced hepatocellular carcinoma (HCC) with portal vein tumor thrombus (PVTT). Methods: The present study included patients with advanced HCC and PVTT treated with one of the above combination therapies. The downstaging rate, objective response rate (ORR), progression-free survival (PFS), overall survival (OS), disease control rate (DCR), and adverse events (AEs) were assessed. Results: Sixty-two patients were analyzed. The ORR was elevated in the SITS group compared with the ST group (p = 0.036), but no differences were found in DCR (p = 0.067). The survival analysis revealed higher PFS (p = 0.015) and OS (p = 0.013) in the SITS group, with median PFS and OS times of 10.4 and 13.8 months, respectively. Ten patients displayed successful downstaging and underwent surgery in the SITS group, vs. none in the ST group. The prognosis was better in surgically treated patients compared with the non-surgery subgroup, based on PFS (p < 0.001) and OS (p = 0.003). Despite a markedly higher rate of AEs in the SITS group (p = 0.020), including two severe AEs, the SITS combination therapy had an acceptable safety profile. Conclusion: The SITS combination therapy yields higher PFS and OS than the combined administration of sorafenib and TACE in patients with advanced HCC and PVTT, especially as a downstaging strategy before surgery.