Purpose: The role of antiviral prophylaxis in preventing hepatitis B virus (HBV) reactivation before rituximab-based chemotherapy in patients with lymphoma and resolved hepatitis B is unclear. Patients and Methods: Eighty patients with CD20+ lymphoma and resolved hepatitis B were randomly assigned to receive either prophylactic entecavir (ETV) before chemotherapy to 3 months after completing chemotherapy (ETV prophylactic group, n = 41) or to receive therapeutic ETV at the time of HBV reactivation and hepatitis B surface antigen (HBsAg) reverse seroconversion since chemotherapy (control group, n = 39). Results: Fifty-eight patients (72.5%) were positive for hepatitis B surface antibody, and HBV DNA was undetectable in 50 patients (62.5%). During a mean 18-month follow-up period, one patient (2.4%) in the ETV prophylactic group and seven patients (17.9%) in the control group developed HBV reactivation (P = .027). The cumulative HBV reactivation rates at months 6, 12, and 18 after chemotherapy were 8%, 11.2%, and 25.9%, respectively, in the control group, and 0%, 0%, and 4.3% in the ETV prophylactic group (P = .019). Four patients (50%) in the control group had HBsAg reverse seroconversion after HBV reactivation. The cumulative HBsAg reverse seroconversion rates at months 6, 12, and 18 since chemotherapy were 0%, 6.4%, and 16.3% in the control group, respectively, which were significantly higher than those in the ETV prophylactic group (P = .032). Patients with detectable or undetectable viral load could develop HBV reactivation and HBsAg reverse seroconversion. Conclusion: Undetectable HBV viral load before chemotherapy did not confer reactivation-free status. Antiviral prophylaxis can potentially prevent rituximab-associated HBV reactivation in patients with lymphoma and resolved hepatitis B.