We describe a novel species isolated from walnut () which comprises non-pathogenic and pathogenic strains on walnut. The isolates, obtained from a single ornamental walnut tree showing disease symptoms, grew on yeast extract-dextrose-carbonate agar as mucoid yellow colonies characteristic of species. Pathogenicity assays showed that while strain CPBF 424 causes disease in walnut, strain CPBF 367 was non-pathogenic on walnut leaves. Biolog GEN III metabolic profiles disclosed some differences between strains CPBF 367 and CPBF 424 and other xanthomonads. Multilocus sequence analysis with seven housekeeping genes (, , , , , , ) grouped these strains in a distinct cluster from pv. and closer to and pv. . Average nucleotide identity (ANI) analysis results displayed similarity values below 93 % to strains. Meanwhile ANI and digital DNA-DNA hybridization similarity values were below 89 and 50 % to non- strains, respectively, revealing that they do not belong to any previously described species. Furthermore, the two strains show over 98 % similarity to each other. Genomic analysis shows that strain CPBF 424 harbours a complete type III secretion system and several type III effector proteins, in contrast with strain CPBF 367, shown to be non-pathogenic in plant bioassays. Taking these data altogether, we propose that strains CPBF 367 and CPBF 424 belong to a new species herein named sp. nov., with CPBF 424 (=LMG 31037=CCOS 1891=NCPPB 4675) as the type strain.