Objective: Intravenous methylprednisolone is the standard treatment for a multiple sclerosis relapse; however, this fails to improve symptoms in up to one quarter of patients. Immunoadsorption is an accepted treatment for refractory relapses, but prospective comparator-controlled studies are missing. Methods: In this observational study, patients with steroid-refractory acute multiple sclerosis relapses receiving either six courses of tryptophan-immunoadsorption or double-dose methylprednisolone therapy were analysed. Outcomes were evaluated at discharge and three months later. Immune profiling of blood lymphocytes and proteomic analysis were performed by multi-parameter flow cytometry and Olink analysis, respectively (NCT04450030). Results: 42 patients were enrolled (methylprednisolone: 26 patients; immunoadsorption: 16 patients). For determination of the primary outcome, treatment response was stratified according to relative function system score changes (“full/best” vs. “average” vs. “worse/none”). Upon discharge, the adjusted odds ratio for any treatment response (“full/best” + ”average” vs. “worse/none”) was 10.697 favouring immunoadsorption (p = 0.005 compared to methylprednisolone). At follow-up, the adjusted odds ratio for the best treatment response (“full/best” vs. “average” + ”worse/none”) was 103.236 favouring IA patients (p = 0.001 compared to methylprednisolone). Similar results were observed regarding evoked potentials and quality of life outcomes, as well as serum neurofilament light-chain levels. Flow cytometry revealed a profound reduction of B cell subsets following immunoadsorption, which was closely correlated to clinical outcomes, whereas methylprednisolone had a minimal effect on B cell populations. Immunoadsorption treatment skewed the blood cytokine network, reduced levels of B cell-related cytokines and reduced immunoglobulin levels as well as levels of certain coagulation factors. Interpretation: Immunoadsorption demonstrated favourable outcomes compared to double-dose methylprednisolone. Outcome differences were significant at discharge and follow-up. Further analyses identified modulation of B cell function as a potential mechanism of action for immunoadsorption, as reduction of B cell subsets correlated with clinical improvement.