Objective: To investigate the efficacy of rituximab as rescue therapy in patients with relapsing-remitting multiple sclerosis (RRMS) and persistent disease activity confirmed by magnetic resonance imaging (MRI) despite immunosuppressive disease-modifying therapy (DMT). Methods: In this observational nationwide retrospective multicenter study, we first identified 351 off-label rituximab-treated patients through a cohort of 15,984 RRMS patients. In this group, we identified patients with disease activity prior to rituximab confirmed by MRI (one or more new T2 lesion and/or gadolinium-enhancing lesion) despite immunosuppressive DMT (fingolimod, natalizumab, or mitoxantrone) with a follow-up after rituximab initiation longer than 6 months. Outcome data were collected from the French Observatory of Multiple Sclerosis (OFSEP) register and medical charts. Results: A total of 50 patients were identified. Median rituximab treatment duration was 1.1 (0.5–6.4) year. Mean annualized relapse rate significantly decreased from 0.8 during last immunosuppressive DMT to 0.18 after rituximab (p < 0.0001). While 72% of patients showed gadolinium-enhancing lesions on the last MRI performed during last immunosuppressive DMT, 8% of them showed gadolinium-enhancing lesions on the first MRI performed 6.1 (range 1.4–18.4) months after rituximab (p < 0.0001). Conclusion: This study provides level IV evidence that rituximab reduces clinical and MRI disease activity in patients with active RRMS despite immunosuppressive DMT.