An obligately piezophilic bacterium was isolated from sediment collected from the bottom of the Japan Trench at a depth of 7542 m. The isolated strain, designated K41G(T), was closely affiliated with members of the genus Psychromonas on the basis of 16S rRNA gene sequence analysis. Levels of DNA-DNA relatedness between strain K41G(T) and Psychromonas reference strains were significantly lower than that accepted as the phylogenetic definition of a species. The optimal temperature and pressure for growth of strain K41G(T) were 6 degrees C and 60 MPa, respectively. The DNA G+C content was 39.1 mol%. Whole-cell fatty acids consisted of significant amounts of unsaturated fatty acids C(16 : 1) (37 %) and C(14 : 1) (17 %), saturated fatty acid C(16 : 0) (31 %) and polyunsaturated fatty acid C(22 : 6) (8 %). Based on the taxonomic differences observed, strain K41G(T) is considered to represent a novel obligately piezophilic Psychromonas species. The name Psychromonas hadalis (type strain, K41G(T)=JCM 11830(T)=ATCC BAA-638(T)) is proposed. This is the second species of obligately piezophilic bacteria to be proposed in the genus Psychromonas.