Coinfection with human immunodeficiency virus (HIV) and hepatitis B virus (HBV) is a significant clinical challenge, especially during changes in antiretroviral therapy. Tenofovir is a key drug for the simultaneous suppression of HIV and HBV, and its suspension can lead to HBV reactivation in individuals with latent or occult infection. This study reports a case of hepatitis B reactivation after tenofovir withdrawal. The patient was a 68-year-old male diagnosed with HIV in 1985, on antiretroviral therapy since 1998, and with an undetectable HIV viral load for several years. In February 2024, tenofovir disoproxil fumarate was discontinued due to renal changes and osteopenia. Three months after withdrawal, the patient presented with a significant elevation of liver transaminases. Initially, drug-induced hepatotoxicity was considered; however, investigation revealed HBsAg, total anti-HBc, and HBeAg reactants, in addition to an HBV viral load of 16,100,000 copies, confirming viral reactivation. The introduction of tenofovir alafenamide resulted in a progressive reduction in HBV viral load and laboratory improvement. The case highlights the need for complete serological evaluation for hepatitis B before changes in the antiretroviral regimen and the risk associated with inadvertent suspension of tenofovir in coinfected patients.