Background The treatment of hepatocellular carcinoma (HCC) requires diverse and multidisciplinary approaches. In recent years, new agents with good antitumor effects have emerged for systemic chemotherapy, and conversion surgery (CS) after systemic chemotherapy is expected to be an effective treatment strategy for unresectable HCC. We herein report a case of unresectable HCC with portal vein tumor thrombus (PVTT) in which atezolizumab plus bevacizumab therapy induced PVTT regression, followed by CS with R0 resection. Case presentation The patient was a 79-year-old man with S2/S3 HCC who was referred to our department due to tumor re-growth and PVTT after two rounds of transcatheter arterial chemoembolization. The PVTT extended from the left portal vein to the main trunk, and it was determined that the resection of the left portal vein would be difficult to perform with R0 status. Based on the diagnosis of unresectable HCC, treatment with atezolizumab plus bevacizumab was initiated. After two courses of treatment, contrast-enhanced computed tomography showed that the PVTT had regressed to the peripheral side of the left portal vein, and R0 resection became possible. The patient developed grade 3 skin lesions as an immune-related adverse event, and it was determined that the continuation of chemotherapy would be difficult. Four weeks after the second course of atezolizumab plus bevacizumab administration, left lobectomy was performed. Intraoperative ultrasonography was used to confirm the location of the tumor thrombus in the left portal vein during the resection, and a sufficient surgical margin was obtained. The histopathological findings showed that primary tumor and PVTT were mostly necrotic with residues of viable tumor cells observed in some areas. The liver background was determined as A1/F4 (new Inuyama classification). The resection margins were negative, and R0 resection was confirmed. There were no postoperative complications. No recurrence was observed as of five months after surgery. Conclusions CS with atezolizumab plus bevacizumab therapy has potential utility for the treatment of unresectable HCC with PVTT.