Inclusion Criteria: 1. Male ≥ 18 years of age. 2. Moderate/severe or severe hemophilia B (baseline FIX activity ≤ 2% of normal or documented history of FIX activity ≤2%). 3. At least 3 bleeding episodes per year that require on-demand treatment with FIX OR are treated with a prophylactic regimen of FIX. 4. At least 100 days exposure history to FIX. 5. No documented history of inhibitors (neutralizing antibodies) to exogenous FIX. 6. No known allergic reaction to exogenous FIX or any component of DTX101. 7. Willing to stop prophylactic treatment with recombinant FIX at specified time points during the study. Exclusion Criteria: 1. History of significant liver disease (ie, portal hypertension). 2. Significant hepatic inflammation or cirrhosis. 3. Evidence of active hepatitis B virus (HBV) or hepatitis C virus (HCV) infection. 4. History of human immunodeficiency virus (HIV) infection AND any of the following: CD4+ cell count \< 350 cells/mm\^3, change in antiretroviral therapy regimen within 6 months prior to Day 0, or plasma viral load \> 200 copies/mL, on 2 separate occasions, as measured by polymerase chain reaction. 5. Anti-AAVrh10 neutralizing antibody titer \> 1:5. 6. Participation (current or previous) in another gene therapy study. 7. Participation in another investigational medicine study within 3 months before screening. NOTE: Other protocol defined inclusion/exclusion criteria may apply.