An anaerobic bacterial strain, designated MA18T, was isolated from a laboratory-scale biogas fermenter fed with maize silage. Cells stained Gram-negative and performed Gram-negative in the KOH test. The peptidoglycan type was found to be A1y-meso-Dpm direct. The major cellular fatty acids were C14 : 0 iso, C15 : 0 iso, anteiso and iso DMA as well as a C16 unidentified fatty acid. Oxidase and catalase activities were absent. Cells were slightly curved rods, motile, formed spores and measured approximately 0.35 µm in diameter and 3.0–5.0 µm in length. When cultivated on GS2 agar with cellobiose, round, arched, shiny and slightly yellow-pigmented colonies were formed. The isolate was mesophilic to moderately thermophilic with a growth optimum between 40 and 48 °C. Furthermore, neutral pH values were preferred and up to 1.2 % (w/v) NaCl supplemented to the GS2 medium was tolerated. Producing mainly acetate and ethanol, MA18T fermented arabinose, cellobiose, crystalline and amorphous cellulose, ribose, and xylan. The genome of MA18T consists of 4 817 678 bp with a G+C content of 33.16 mol%. In the annotated protein sequences, cellulosomal components were detected. Phylogenetically, MA18T is most closely related to Ruminiclostridium sufflavum DSM 19573T (76.88 % average nucleotide identity of the whole genome sequence; 97.23 % 16S rRNA gene sequence similarity) and can be clustered into one clade with other species of the genus Ruminiclostridium , family Oscillospiraceae , class Clostridia . Based on morphological, physiological and genetic characteristics, this strain represents a novel species in the genus Ruminiclostridium . Therefore, the name Ruminiclostridium herbifermentans sp. nov. is proposed. The type strain is MA18T (=DSM 109966T=JCM 39124T).