Lassa virus spreads from a rodent to humans and can lead to lethal hemorrhagic fever. Despite its broad tropism, chicken cells were reported 30 years ago to resist infection. We found that Lassa virus readily engaged its cell-surface receptor a-dystroglycan in avian cells, but virus entry in susceptible species involved a pH-dependent switch to an intracellular receptor, the lysosome-resident protein LAMP1. Iterative haploid screens revealed that the sialyltransferase ST3GAL4 was required for the interaction of the virus glycoprotein with LAMP1. A single glycosylated residue in LAMP1, present in susceptible species but absent in birds, was essential for interaction with the Lassa virus envelope protein and subsequent infection. The resistance of Lamp1-deficient mice to Lassa virus highlights the relevance of this receptor switch in vivo.