The Staniantsi-Mazgoš Basin is one of several Neogene intramontane basins in NW Bulgaria. Recent fieldwork in the open pit coal mine yielded material of an exceptional diversity of vertebrates from the uppermost Miocene. In particular, skeletal remains of a large Castorinae are very numerous and well-preserved. Here we perform a comparative morphological description of the postcranium of the Staniantsi-beaver and compare it with the extant Castor fiber and closely related fossil taxa, noting an overall high degree of similarity in many cases. Analyses of the functional anatomy confirm similar locomotor adaptations of the large Staniantsi-beaver and the extant Castor fiber. It is shown that the hindlimb exhibits typical adaptations for swimming and the forelimb is modified for a primary fossorial movement. Further, the caudal vertebrae indicate a flattened tail. Minor osteological differences can be used in a future clarification of the taxonomic status of this fossil castorine. In conclusion, the large Staniantsi-beaver seems to be equivalent to the extant Castor fiber and several fossil castorids in his locomotor adaptations and fits perfectly into the assumed swampy to lacustrine palaeoenvironment of the Staniantsi-Mazgoš Basin. Our results highlight the importance of the postcranium for reconstructing the palaeobiology and elucidating the taxonomy of fossil rodents.