Biosimilar drugs-lower-cost alternatives to expensive biologic drugs-have the potential to slow the growth of US drug spending. However, rates of biosimilar uptake have varied across hospital outpatient providers. We investigated whether the 340B Drug Pricing Program, which offers eligible hospitals substantial discounts on drug purchases, inhibits biosimilar uptake. Almost one-third of US hospitals participate in the 340B program. Using a regression discontinuity design and two high-volume biologics with biosimilar competitors, filgrastim and infliximab, we estimated that 340B program eligibility was associated with a 22.9-percentage-point reduction in biosimilar adoption. In addition, 340B program eligibility was associated with 13.3 more biologic administrations annually per hospital and $17,919 more biologic revenue per hospital. Our findings suggest that the program inhibited biosimilar uptake, possibly as a result of financial incentives making reference drugs more profitable than biosimilar medications.