Abstract Introduction Prospective data on the risk of hepatitis B reactivation (HBVr) among patients with resolved HBV infection undergoing anti-CD20 antibodies monotherapy is scarce. We aimed to assess the risk of HBVr in patients with resolved HBV infection treated with rituximab or ocrelizumab in monotherapy for multiple sclerosis (MS) or neuromyelitis optica spectrum disorder (NMOSD) without antiviral prophylaxis. Methods HEBEM is a prospective study that included all consecutive adults HBsAg-negative/anti-HBc-positive who initiated anti-CD20 antibodies for MS or NMOSD at Cemcat. Inclusion criteria encompassed undetectable HBV-DNA, absence of other immunosuppressants or antiviral therapy. Every 6 months HBsAg, ALT and HBV-DNA were performed to rule out HBVr (defined by 2-log increase in HBV-DNA or seroconversion to HBsAg+). Results From August/2019 to August/2022, 540 subjects initiated anti-CD20 antibodies, 28 (5.2%) were anti-HBc-positive and were included. Twenty-two received rituximab and 6 ocrelizumab. The majority (89.3%) had previously received ≥ 1 immunomodulatory drug, with corticosteroids (82.1%) and interferon (42.9%) as the most common. At inclusion, all presented normal transaminases and undetectable HBV-DNA. Median anti-HBs levels were 105.5 mIU/mL (IQR 0–609). Median follow-up was 3.1 years (2.1–4.0). Median number of cycles of anti-CD20 antibodies was 6 (3–7), with a cumulative dose of 8.5 g (5.8–11.2) of rituximab and 3 g (1.8–3.8) of ocrelizumab. Neither cases of HBVr nor changes in anti-HBs titers were observed per 83.6 patient-years treated with monotherapy with anti-CD20 antibodies. Conclusions In this cohort of patients with MS or NMOSD and resolved HBV infection, anti-CD20 monotherapy was not associated with detectable risk of HBV reactivation despite the lack of antiviral prophylaxis.