In the present paper, we describe the craniodental remains of three individuals of an amphicyonid previously determined as Thaumastocyon sp. from the late Miocene (c. 9.1 Ma) pseudokarstic site of Batallones-3. Dentognathic differences in relation to other Thaumastocyoninae enable a new taxon, Ammitocyon kainos gen. et sp. nov., to be defined; it is both the most recent and the most complete member of this subfamily known in the fossil record. The results of our phylogenetic analysis demonstrate that this new form reached the maximum degree of specialization within Thaumastocyoninae, a group that includes the most hypercarnivorous amphicyonid species of the Miocene. The masticatory apparatus of A. kainos is extremely derived, with the loss of the mesial premolars (P3 / P2 and p1-p3) and the third molars (M3 / m3). The robustness of the chin and muzzle is in contrast with the slender and highly sectorial postcanine dentition (p4 / m2 and P4 / M2), features that are consistent with the values provided by an analysis of the bending resistance of the mandible performed for these specimens. All the anatomical features combined, both cranial and postcranial, reveal the complexity of the body plan of A. kainos. This species combines a derived hypercarnivorous dentition with one of the most robust postcranial skeletons recorded for all large caniform carnivorans, outlining an enigmatic taxon that possesses unique ecological adaptations.