Severely infected walnut trees were observed in six regions of South Korea via a survey of tree diseases that was carried out to identify tree pathogens between 2016 and 2017. Disease symptoms included shoot blight, canker on the branches or twigs and one to several black lesions that appeared on catkins, leaves and nuts. Over 300 bacterial colonies were isolated from infected tissues using nutrient agar medium. Of those, isolates that had rod-shaped colonies, yellow pigments on NA medium and fluorescent pigments produced under UV when grown in King’s B agar medium, which is typical characteristics of Pseudomonas species, were selected (King et al. 1954). Six representative Pseudomonas species, representing each geographical region, were randomly selected and sequenced. All the sequences generated were deposited to the NCBI database (accession Nos. MH709127, MH714738–42). To ensure the identity of the Pseudomonas species, three housekeeping genes and 16S rRNA gene region were combined (Hilario et al. 2004), aligned and subsequently used to construct a phylogenetic tree based on a Maximum likelihood analysis. The phylogenetic tree resulted in a supported placement of the Pseudomonas strains obtained in this study (strain NIFOS2) with authenticated isolates of P. flavescens retrieved from GenBank, confirming its identification as P. flavescens. To confirm the pathogenicity, the bacterial suspension made from the strain NIFOS2 was inoculated to the leaves and fruits wounded by a sterile needle. The tested leaves and fruits were placed at 28 °C for 7 days. Necrotic lesions and black spots were observed in 7 days after the inoculation, but not on the controls treated with sterile water. Re-isolations were successfully made from the inoculated leaves and fruits, fulfilling the Koch’s postulates. To our knowledge, this is the first report of walnut blight canker on walnut trees (J. regia) caused by P. flavescens in South Korea.