Mesenchytraeus monodiverticulus, new species, (Clitellata: Enchytraeidae) is described based on live and preserved specimens from Changbai Mountain. It is diagnosed by possessing enlarged ventral chaetae in V–VI (4–5 per bundle), nephridium holding solid unlobed postseptale, spermathecae possessing only one diverticulum and attached entally to esophagus in VII, atria cylindrical without prostate glands, and the presence of regular spermatozoal bundles in seminal vesicle (XI–XIII) with the heads of spermatozoa arranged in both ends orderly. The major differences between M. monodiverticulus and its congeners are discussed. A key to species of Mesenchytraeus that possess enlarged chaetae is provided.