Hepatitis B virus (HBV) reactivation (HBVr) represents a severe and potentially life-threatening condition, and preventive measures are available through blood test screening or prophylactic therapy administration. The assessment of HBVr traditionally considers factors such as HBV profile, including hepatitis B surface antigen (HBsAg) and antibody to hepatitis B core antigen, along with type of medication (chemotherapy; immunomodulants). Nevertheless, consideration of possible patient's underlying tumor and the specific malignancy type (solid or hematologic) plays a crucial role and needs to be assessed for decision-making process.