Background: Rituximab-abbs is a CD20-directed monoclonal antibody and the first rituximab biosimilar approved in the US, expected to significantly reduce drug acquisition costs. This budget impact model (BIM) estimated budgetary impact of replacing a proportion of rituximab/hyaluronidase human subcutaneous injection (SC-R) utilization for NHL (diffuse large B-cell lymphoma [DLBCL], follicular lymphoma [FL]) and CLL with rituximab-abbs (IV-R-BIOSIM). The objective was to project incremental cost differences between IV-R-BIOSIM and SC-R over one year for a hypothetical 5-million-member US healthcare insured (Medicare) population. Methods: An illustrative BIM was developed to estimate 1-year drug and administration costs for a one-quarter shift in SC-R market utilization to IV-R-BIOSIM, with equal efficacy and safety assumed. Values for epidemiology, market share, drug dosing, administration, and costs were derived from scientific literature, product labels, and publicly available cost resources. Infused rituximab (IV-R) dosing assumed a mean body surface area (BSA) of 1.8m². Annual dose counts of IV-R-BIOSIM or SC-R per patient were: 10 untreated FL with maintenance; 8 untreated FL (without maintenance), relapsed/refractory FL, or untreated DLBCL; 6 CLL. IV-R infusion duration was 3 hours. Drug acquisition and infusion/subcutaneous administration costs were from 2020 Average Sales Price pricing file and Centers for Medicare and Medicaid Services Physician Fee Schedule. SC-R costs included an initial IV-R originator dose. Patient cost share was based on 2020 Medicare Part B 20% cost-share for office visits and drugs. Univariate sensitivity analyses were conducted. A scenario analysis used 2-year dosing to estimate costs for further FL maintenance treatment. Results: For a 5-million-member insured population, an estimated 972 patients would receive rituximab for NHL or CLL; 49 would receive SC-R. Estimated total incremental savings for one year for a 13-patient shift from SC-R to IV-R-BIOSIM were $57,864, equating to $0.02 per enrolled member per year (PMPY). Per-patient incremental annual savings with IV-R-BIOSIM for one year ranged between $2,359–$8,186 (Table). The model was most sensitive to low or high BSA dosing and proportion of patients with CLL. Conclusions: This BIM estimated annual savings of over $57,000 ($0.02 PMPY) for a 5-million-member US payer following a 25% shift of current SC-R use to IV-R-BIOSIM. These findings demonstrate the potential economic benefits of IV-R-BIOSIM vs SC-R that may result in expanded access to rituximab therapy.