BACKGROUND Immune reconstitution inflammatory syndrome (IRIS) is a well-recognized complication after antiretroviral therapy (ART) initiation among patients with HIV. Acute HBV flares after starting antiretroviral therapy have been reported in 20% to 25% of coinfected patients, among whom only 1% to 5% develop clinical hepatitis. Liver biopsy and serological evaluation help in diagnosis. CASE REPORT A 24-year-old man with history of HIV diagnosed in 2018 developed severe IRIS-related HBV flare after initiation of ART. He was taking ART since 2018 until his immigration to the United States in 2021. He came to establish care and was started on bictegravir/emtricitabine/tenofovir alafenamide (BIC/F/TAF). Three weeks later, he presented to the Emergency Department with polyarthralgia and loose stools; transaminases showed an increasing trend on follow-up. He was admitted for closer monitoring. Workup was remarkable for reactive HBsAg, HBeAg, and HBcIgM antibodies, with HBV viral load of 295 304 copies/mL. Abdominal imaging was unremarkable. ART was switched to rilpivirine/emtricitabine/tenofovir alafenamide (RPV/FTC/TAF), considering the hypothetical risk of hepatotoxicity from BIC/F/TAF. Despite therapy, transaminases were up-trending. He underwent computerized tomography-guided liver biopsy, showing moderate to severe acute hepatitis, compatible with IRIS. He received steroids, and ART was continued. Transaminases resolved, HBV load reduced significantly, HIV load became undetectable at 9 weeks, and he developed HBeAb (seroconversion) at 4 months after initiating ART. CONCLUSIONS Our case highlights the importance of early recognition and management of IRIS-HBV flares after initiation of ART among coinfected patients. Liver biopsy is indicated for definitive diagnosis. ART directed against both viruses should be continued.