The family Coronaviridae represents a diverse group of vertebrate RNA viruses, all with genomes greater than 26,000 nucleotides. Here, we report the discovery and genetic characterization of a novel virus present in cattle with respiratory disease. Phylogenetic characterization of this virus revealed that it clusters within Torovirinae, a subfamily of Coronaviridae. The complete genome consists of only 20,261 nucleotides, and represents the smallest reported Coronaviridae genome. We identified seven open reading frames, including the canonical nidovirus open reading frames 1a and b. Analysis of polyprotein 1ab revealed that this virus, tentatively named bovine nidovirus (BoNV), shares the highest homology with the recently described python-borne nidoviruses and contains several conserved nidovirus motifs, but does not encode the NendoU or O-MT domains that are present in other viruses within Coronaviridae. In concert with its reduced genome, the atypical domain architecture indicates that this virus represents a unique lineage within the order Nidovirales.