A Gram-staining-negative, rod-shaped, non-spore-forming bacterium, designated strain E103(T), was isolated from the skin of the medical leech Hirudo verbana. 16S rRNA gene sequence analysis showed that the isolate was closely related to species of the genus Castellaniella. Castellaniella ginsengisoli DCY36(T) was shown to be the most closely related (98.4 % 16S rRNA gene sequence similarity), followed by Castellaniella denitrificans NKNTAU(T) and Castellaniella daejeonensis MJ06(T) (both 97.8 %), then Castellaniella caeni Ho-11(T) (97.5 %). Chemotaxonomic data (major ubiquinone, Q-8; major polar lipids, phosphatidylethanolamine, phosphatidylglycerol and phosphatidylserine; predominant polyamine, putrescine with a moderate amount of 2-hydroxyputrescine; and major fatty acids, C(17 : 0) cyclo, C(16 : 0) and summed feature 4 comprising C(16 : 1)ω7c and/or iso-C(15 : 0) 2-OH) supported the affiliation of the isolate to the genus Castellaniella. DNA-DNA hybridization values with the type strains of all species of the genus Castellaniella were 23 % (reciprocal, 18 %) with C. ginsengisoli KCTC 22398(T), 20 % (26 %) with C. daejeonensis KCTC 22454(T), 11 % (58 %) with C. denitrificans DSM 11046(T) and 13 % (12 %) with C. caeni KCTC 12197(T)(.) Phenotypic differentiation of strain E103(T) from its closest neighbours was possible. Strain E103(T) therefore represents a novel species of the genus Castellaniella, for which the name Castellaniella hirudinis sp. nov. is proposed, with the type strain E103(T) ( = CCUG 62394(T) = LMG 26910(T)).