Bacterial strain YM22-133^T was isolated from the sediment of an estuary and grew in media with an artificial seawater base. Strain YM22-133^T was Gram-positive, aerobic, non-motile and rod shaped. The cell-wall peptidoglycan contained LL-DAP, glycine, alanine and hydroxyglutamate. The predominant menaquinone was MK-9 (H₈), with MK-9 (H₀), MK-9 (H₂), MK-9 (H₄) and MK-9 (H₆) present as minor menaquinones. The G+C content of the genomic DNA from the strain was 68 mol%. Phylogenetic analysis of the 16S rRNA gene sequence showed that the strain is nearest to Acidimicrobium ferrooxidans DSM 10331^T. However, the similarity is relatively low (87.1%) and the physiological characteristics are also different: Acidimicrobium ferrooxidans is thermotolerant and acidophilic. Therefore, strain YM22-133^T can be classified as a novel genus and species, Ilumatobacter fluminis gen. nov., sp. nov. (type strain YM22-133^T =DSM 18936^T=MBIC08263^T).