Biosimilars provide an opportunity for a more sustainable and cost-effective treatment for multiple sclerosis (MS). This study evaluated the potential financial impact of implementing a formulary change from reference to biosimilar natalizumab (NTZ) from the US commercial payer perspective.The budget impact of transitioning to biosimilar NTZ for the treatment of relapsing-remitting MS (RRMS) was estimated over a 3-year time horizon based on real-world dosing. Additional scenario analyses were conducted by varying the price differential of biosimilar NTZ.The target population was estimated from a 1-million-member hypothetical commercial health plan. Model inputs were drug acquisition costs and treatment-related and patient coinsurance costs. Budget impact and cost savings per member per year were calculated by assuming a biosimilar uptake of 10% in year 1 to 20% in year 3.Over 3 years, 255 patients were estimated to be treated with high-efficacy disease-modifying therapies for RRMS. The inclusion of biosimilar NTZ onto a formulary would result in cumulative cost savings to payers of $452,611 over 3 years, with mean savings per treated member per year of $1179, $1769, and $2359 in years 1, 2, and 3, respectively. One-way sensitivity analyses indicated that budget impact results were most sensitive to drug acquisition costs of both reference and biosimilar NTZ.Adoption of biosimilar NTZ can yield considerable cost savings to US health plans that could result in increased treatment access for patients with RRMS.