Endophytic bacterial communities of tidal flat plants antagonistic to oomycete plant pathogens were studied by the isolation of 256 root colonizing endophytic bacteria from surface-disinfected root tissues of six plants (Rosa rugosa, Suaeda maritima, Vitex rotundifolia, Carex scabrifolia, Glehnia littoralis and Elymus mollis) growing in a tidal flat area of Namhae Island, Korea. To understand the antagonistic potential, an in vitro antagonistic assay was performed to characterize and identify strains that were antagonistic to the oomycete plant pathogens Phytophthora capsici and Pythium ultimum from the total population. Nine percent of the total number of isolated bacteria exhibited in vitro inhibitory activity against target plant pathogenic oomycetes. Taxonomic and phylogenetic placement of the antagonistic bacteria was investigated by analysis of the 16S rRNA gene sequences. The sequence analysis classified the antagonistic strains into four major classes of the domain bacteria (Firmicutes, α-Proteobacteria, γ-Proteo-bacteria and Actinomycetes) and 10 different genera. Further production of secondary metabolites, hydrolytic enzymes and plant growth promoting traits were determined for the putative new species of antagonistic endophytic bacteria. These new strains could not be identified as known species of α-Proteobacteria, and so may represent novel bacterial taxa. The unexpected high antagonistic bacterial diversity associated with the tidal flat plants may be indicative of their importance in tidal flat plants as a promising source of novel antimicrobial compounds and biocontrol agents.