Background: Rituximab-abbs is an anti-CD20 monoclonal antibody and an important immuno-oncology agent for the treatment of B-cell malignancies NHL (diffuse large B-cell lymphoma [DLBCL] and follicular lymphoma [FL]) and CLL. It is also indicated for patients with RA, GPA, and MPA. Rituximab-abbs was the first rituximab biosimilar approved in the US and is expected to reduce drug acquisition costs. This budget impact model (BIM) estimated the impact of replacing a share of originator rituximab (IV-R-REF) use with rituximab-abbs (IV-R-BIOSIM) for NHL (DLBCL and FL), CLL, RA, GPA, and MPA. The objective was to project incremental annual cost differences between IV-R-BIOSIM and IV-R-REF for a hypothetical 1-million-member US healthcare insured (Medicare) population. Methods: An illustrative BIM estimated changes in 1-year drug and administration costs for an increased IV-R-BIOSIM uptake from 17.5% to 22.0%. Values for epidemiology, market share distribution, drug dosing, administration, and costs were derived from scientific literature, product labels, and publicly available cost resources. Dosing was based on a mean patient body surface area of 1.8 m². Annual dose counts per patient were: 10 untreated FL with maintenance; 8 untreated FL (without maintenance), relapsed/refractory FL, or untreated DLBCL; 6 CLL, and 4 for RA, GPA, or MPA. All treatments were assumed to infuse over 3 hours. Drug acquisition and administration costs were from 2020 Average Sales Price pricing file and Centers for Medicare and Medicaid Services Physician Fee Schedule. Patient cost share was based on 2020 Medicare Part B 20% cost-share for office visits and drug products. Univariate sensitivity analyses were conducted. A scenario analysis was performed to project 2-year costs for extended FL maintenance treatment. Results: Estimated total annual plan incremental savings for a 1-million-member payer after the utilization shift were $312,379, equating to $0.31 per enrolled member per year (PMPY). Per-patient incremental drug cost savings with IV-R-BIOSIM for 1-year were $5,474–$12,924 (Table). The model was most sensitive to IV-R-REF cost and proportion of patients with RA. Conclusions: This analysis estimated annual savings of over $310,000 ($0.31 PMPY) for a 1-million-member US payer following a 4.5% utilization shift from IV-R-REF to IV-R-BIOSIM, demonstrating that IV-R-BIOSIM may confer considerable economic benefits vs originator rituximab.