Infectious bronchitis virus (IBV) infects ciliated epithelial cells in the chicken respiratory tract. While some IBV strains replicate locally, others can disseminate to various organs, including the kidney. Here, we elucidate the determinants for kidney tropism by studying interactions between the receptor-binding domain (RBD) of the viral attachment protein spike from two IBV strains with different tropisms. Recombinantly produced RBDs from the nephropathogenic IBV strain QX and from the nonnephropathogenic strain M41 bound to the epithelial cells of the trachea. In contrast, only QX-RBD binds more extensively to cells of the digestive tract, urogenital tract, and kidneys. While removal of sialic acids from tissues prevented binding of all proteins to all tissues, binding of QX-RBD to trachea and kidney could not be blocked by preincubation with synthetic alpha-2,3-linked sialic acids. The lack of binding of QX-RBD to a previously identified IBV-M41 receptor was confirmed by enzyme-linked immunosorbent assay (ELISA), demonstrating that tissue binding of QX-RBD is dependent on a different sialylated glycan receptor. Using chimeric RBD proteins, we discovered that the region encompassing amino acids 99 to 159 of QX-RBD was required to establish kidney binding. In particular, QX-RBD amino acids 110 to 112 (KIP) were sufficient to render IBV-M41 with the ability to bind to kidney, while the reciprocal mutations in IBV-QX abolished kidney binding completely. Structural analysis of both RBDs suggests that the receptor-binding site for QX is located at a different location on the spike than that of M41.IMPORTANCE Infectious bronchitis virus is the causative agent of infectious bronchitis in chickens. Upon infection of chicken flocks, the poultry industry faces substantial economic losses by diminished egg quality and increased morbidity and mortality of infected animals. While all IBV strains infect the chicken respiratory tract via the ciliated epithelial layer of the trachea, some strains can also replicate in the kidneys, dividing IBV into the following two pathotypes: nonnephropathogenic (example, IBV-M41) and nephropathogenic viruses (including IBV-QX). Here, we set out to identify the determinants for the extended nephropathogenic tropism of IBV-QX. Our data reveal that each pathotype makes use of a different sialylated glycan ligand, with binding sites on opposite sides of the attachment protein. This knowledge should facilitate the design of antivirals to prevent coronavirusinfections in the field.