The primary objective of the study is to evaluate, in participants with RMS, safety and tolerability (as defined by the frequency of adverse events [AEs] of flu-like symptoms [FLS; chills, pyrexia, myalgia, and asthenia], injection site reactions [ISRs], and injection site reaction pain [ISR-P]) over 6 months of treatment (the active comparator period) with BIIB017 125 μg subcutaneously (SC) every 2 weeks versus standard-of-care SC interferon-beta (IFN-β) therapy. Secondary objectives of this study are to assess the following measures during the first (6-month) period of the study in participants treated with BIIB017 versus standard-of-care SC IFN-β therapy: patient-reported treatment satisfaction using the following patient-reported outcome measures (PROMs): Treatment Satisfaction Questionnaire for Medication (TSQM-9), Adapted MS Treatment Concerns Questionnaire (MSTCQ), Adapted MSTCQ Side Effects Score, Pain using a visual analog scale (VAS) diary and the McGill Pain Questionnaire Short Form (SF-MPQ), the treatments' impact on RMS using the following PROMs: Multiple Sclerosis Impact Scale (MSIS-29), Modified Fatigue Impact Scale-5 Item (MFIS-5), EuroQol Group 5-Dimension 3-Level Version (EQ-5D-3L), Health-Related Productivity Questionnaire (HRPQ), Beck Depression Inventory, second edition (BDI-II), participant adherence to study treatment, clinical status as measured by the Expanded Disability Status Scale (EDSS) and relapse activity, safety and tolerability of study treatment after a change in standard-of-care SC IFN-β therapy and the immunogenicity profiles of participants changing from standard-of-care SC IFN-β to BIIB017.