Two Gram-stain-negative and non-flagellated bacteria, YSTF-M3 T and YSTF-M6 T , were isolated from a tidal flat from Yellow Sea, Republic of Korea, and subjected to a polyphasic taxonomic study. Neighbour-joining phylogenetic tree of 16S rRNA gene sequences showed that strains YSTF-M3 T and YSTF-M6 T belong to the genera Kordia and Olleya of the family Flavobacteriaceae , respectively. The 16S rRNA gene sequence similarities between strain YSTF-M3 T and the type strains of Kordia species and between strain YSTF-M6 T and the type strains of Olleya species were 94.1–98.4 and 97.3–98.3 %, respectively. The ANI and dDDH values between genomic sequences of strain YSTF-M3 T and the type strains of five Kordia species and between those of strain YSTF-M6 T and the type strains of three Olleya species were in ranges of 77.0–83.2 and 20.7–27.1 % and 79.4–81.5 and 22.3–23.9 %, respectively. The DNA G+C contents of strain YSTF-M3 T and YSTF-M6 T from genomic sequences were 34.1 and 31.1 %, respectively. Both strains contained MK-6 as predominant menaquinone and phosphatidylethanolamine as only major phospholipid identified. Differential phenotypic properties, together with the phylogenetic and genetic distinctiveness, revealed that strains YSTF-M3 T and YSTF-M6 T are separated from recognized species of the genera Kordia and Olleya , respectively. On the basis of the data presented, strains YSTF-M3 T (=KACC 21639 T =NBRC 114499 T ) and YSTF-M6 T (=KACC 21640 T =NBRC 114500 T ) are considered to represent novel species of the genera Kordia and Olleya , respectively, for which the names Kordia aestuariivivens sp. nov. and Olleya sediminilitoris sp. nov. are proposed.