Background: Suratadenoturev (OBP-301) is an oncolytic adenovirus that harbors a promoter of human telomerase reverse transcriptase (hTERT) and is genetically modified to selectively replicate within and then lyse cancer cells. The aim of this study was to assess the safety and optimal dosage for intratumoral (IT) injection of OBP-301 in patients with advanced hepatocellular carcinoma (HCC). Methods: An open-label, non-comparative, phase I dose-escalation trial was performed in 20 patients who had refractory advanced HCC. OBP-301 was administered to the primary tumor using ultrasound guidance. A single IT injection of 10¹⁰ virus particles (VP) was administered to patients in the initial cohort (Cohort-1), and the subsequent single-dose cohorts received 10¹¹ VP (Cohort-2), 10¹² VP (Cohort-3), and 3×10¹² VP (Cohort-4). A multiple dose cohort (Cohort-5) received 2×10¹² VP × 3 times every 2 weeks. Each of the single-dose cohorts had 3 patients and there was a single escalating dose of OBP-301 from 10¹⁰ to 3×10¹² VP. The multiple-dose cohort had 8 patients, and 6 of them received multiple escalating doses of 2×10¹² VP ×3 times every 2 weeks. Results: There were 18 males and 2 females, and the median age was 59.39 years (range: 48.4–65.9). Patients had good tolerance of the single dose and multiple-dose regimens, and the maximum tolerated dose (MTD) was more than 6×10¹² VP/patient. There was no evidence of toxicity with increasing dose, but there was a greater frequency of treatment-emergent adverse events (TEAEs) in Cohorts 4–5 than in Cohorts 1–3. The most common TEAEs related to OBP-301 were influenza-like illness (30%), pyrexia (15%), and fatigue, decreased platelet count, abdominal distension, and anemia (10% each). The overall intrahepatic mRECIST response occurred in 7 patients (39%) with confirmed stable disease (SD) and 11 (61%) with progressive disease (PD). The best target response occurred in 14 patients (78%) and 4 (22%) of them had PD. There was evidence of OBP-301 replication-dependent dissemination in the blood. Cohorts 4–5 had about 50% greater levels of CD8+ T cells in peripheral blood after OBP-301 injection. Conclusions: Multiple IT injections of OBP-301 are well-tolerated in advanced HCC. Although antitumor activity of the study medication alone would not be demonstrated obviously, SD observed as best local response was higher than as overall response. Improved anti-tumor efficacy can be achieved with adjusting viral injection volume to the target sites as well as with adopting the combination therapy with another immunothrapeutics in further study (JRCT ID: jRCT2033200223). Clinical trial information: NCT02293850.