A Gram-reaction-negative, non-motile and rod-shaped bacterium, designated as THG-DN3.6T, was isolated from an ancient tree trunk from Republic of Korea. On the basis of 16S rRNA gene sequence analysis, strain THG-DN3.6T was shown to belong to the genus Chryseobacterium and the highest similarity to Chryseobacterium indoltheticum LMG 4025T (97.2 %) and the closest phylogenetic relatives were Chryseobacterium scophthalmum (96.8 %), Chryseobacterium piscium (96.7 %) and Chryseobacterium balustinum KCTC 2903T (96.3 %). The DNA G + C content of the isolate was 33.2 mol %. The predominant isoprenoid quinone was menaquinone-6. The major fatty acids were iso-C15:0, summed feature 3 (C16:1 ω7c and/or C16:1 ω7t and/or iso-C15:0 2-OH), iso-C17:1 ω9c and iso-C17:0 3-OH. The major polar lipids of strain THG-DN3.6T were phosphatidylethanolamine. The mean DNA–DNA relatedness of strain THG-DN3.6T to C. indoltheticum LMG 4025T was 52 ± 0.5 %. Based on the results of polyphasic characterization, strain THG-DN3.6T represented a novel species within the genus Chryseobacterium, for which the name Chryseobacterium formosus sp. nov. is proposed. The type strain is THG-DN3.6T (=KCTC 42606 = CCTCC AB 2015118). The NCBI GenBank accession number for the 16S rRNA gene sequence of strain THG-DN3.6T is KM035938.