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(1) Background: We aimed to evaluate the risk of hepatitis B virus (HBV) reactivation in lung cancer patients treated with tyrosine kinase inhibitor (TKI), particularly in those with resolved HBV infection. (2) Methods: In this retrospective hospital-based cohort study, we screened all lung cancer patients with positive hepatitis B core antibodies (anti-HBc) receiving systemic antineoplastic treatment during the period from January 2011 to December 2020. Cumulative incidences of HBV reactivation, and their hazard ratios (HRs), were evaluated after adjusting patient mortality as a competing risk. (3) Results: Among 1960 anti-HBc-positive patients receiving systemic therapy, 366 were HBsAg-positive and 1594 were HBsAg-negative. In HBsAg-positive patients without prophylactic NUC, 3-year cumulative incidences of HBV reactivation were similar between patients receiving chemotherapy and patients receiving TKI (15.0%, 95% confidence interval (CI): 0–31.2% vs. 21.2%, 95% CI: 10.8–31.7%; p = 0.680). Likewise, 3-year cumulative incidences of HBV-related hepatitis were similar between the two groups (chemotherapy vs. TKI: 15.0%, 95% CI: 0–31.2% vs. 9.3%, 95% CI: 2.8–15.7%; p = 0.441). In 521 HBsAg-negative TKI users, the 3-year cumulative incidence of HBV reactivation was only 0.6% (95% CI: 0.0–1.9%). From multivariable regression analysis, we found that the only independent risk factor for HBV reactivation in TKI users was HBsAg positivity (HR 53.8, 95% CI: 7.0–412.9; p < 0.001). (4) Conclusion: Due to high risks of HBV reactivation in HBsAg-positive TKI users, NUC prophylaxis can be considered. However, in patients with resolved HBV infection, such risks are lower, and therefore regular monitoring is recommended. [ABSTRACT FROM AUTHOR]