Knowledge about how Toll-like receptors (TLRs) recognize pathogenic ligands is critical to understanding how these receptors are activated and to designing therapeutic compounds that target this family of receptors for inflammatory diseases. The crystal structure of TLR5 in complex with its bacterial ligand flagellin revealed that the ligand-binding mode for TLR5 is distinct from that of previously characterized TLRs. Nevertheless, like other TLRs, TLR5 forms a dimer in response to ligand binding. This work contributes to our current knowledge of TLR function and further demonstrates the ability of TLRs to couple versatile ligand recognition to a conserved receptor signaling mechanism.