• Each patient eligible to participate in this study must meet all the following criteria:

‣ Written informed consent

⁃ Primary diagnosis of HCC, planned to receive radiation, treatment naïve to systemic therapy for HCC, prior TACE permitted

⁃ Hepatocellular carcinoma diagnosis by histologic findings and/or imaging criteria of LI-RADS 5

⁃ Eastern Cooperative Oncology Group performance status score of 0-2

⁃ Age\>/=18 years

⁃ Child-Pugh class A liver function or B7, BCLC A-C or deemed not a candidate for surgery or liver transplantation

⁃ No extrahepatic metastasis detected on CT chest with or without IV contrast, abdomen and pelvis with IV and oral contrast (triphasic-if feasible based on kidney function), or MRI abdomen/liver and chest CT.

⁃ Females of childbearing potential must be willing to use a highly effective method of birth control for the duration of the study, and ≥ 6 months after the last dose of tislelizumab, and have a negative urine or serum pregnancy test ≤ 7 days of first dose of study drug

⁃ Non-sterile males must be willing to use a highly effective method of birth control for the duration of the study and for ≥ 6 months after the last dose of tislelizumab. Males must agree not to donate or bank sperm during treatment with tislelizumab and for \> 6 months after treatment stop.

‣ Must have 1 target lesion measurable in 1 dimension according to RECIST 1.1.

‣ Demonstrate adequate bone marrow and organ function as defined below:

• Hematologic - Absolute neutrophil count (ANC) ≥ 1,500/mcL, Hemoglobin \> 8.5 g/dL, Platelet count ≥ 75,000/mcL

∙ Renal - Serum creatinine OR calculated\* serum creatinine clearance (GFR can be used in place of creatinine or creatinine clearance) ≤ 1.5x upper limit of normal (ULN) OR ≥ 30 mL/min for participants with creatinine levels \> 1.5x institutional ULN

⁃ Calculate serum creatinine clearance using the standard Cockcroft-Gault formula.

• Urine protein Urine dipstick for proteinuria \< 2+ within 7 days prior to start of study treatment \*Participants with ≥ 2+ proteinuria on dipstick analysis at baseline should undergo a 24-hour urine collection which must demonstrate \< 1g of protein in 24 hours

∙ Hepatic - Serum total bilirubin ≤ 3 mg/dL , AST (SGOT) and ALT (SGPT) ≤ 5x ULN , Alkaline phosphatase (ALP) ≤ 8x ULN Coagulation - International Normalized Ratio (INR) or prothrombin time (PT) or activated partial thromboplastin time (aPTT) ≤ 2.0x ULN \*This applies only to participants not receiving therapeutic anticoagulation; participants receiving therapeutic anticoagulation should be on a stable dose.