A detailed anatomy of the braincase and stapes of the subadult specimen of Diadectes absitus from early Permian sediments of Germany based on the high-resolution X-ray microcomputed tomography are described for the first time. In contrast to previous studies of Diadectes, the bones of the braincase (opisthotic, prootic, supraoccipital, basioccipital, exoccipital, basisphenoid, sphenethmoid), and parasphenoid of D. absitus are not co-ossified, but suturally defined. This has allowed for a reconstruction of a complete braincase with all sutures between the individual bones. The opisthotic, prootic, and supraocciptal contain a well-preserved endosseous labyrinth. The 3D-reconstruction of its cavities shows a well-preserved vestibule, three semicircular canals, and well-developed cochlear recess. In addition, a shallow subarcuate fossa is present on the ventral surface of the supraoccipital, which lies medial to the anterior semicircular canal. A typical feature of the diadectid braincase is the presence of the otic tube leading from the fenestra vestibuli to the vestibule. A revision of the topology of this structure is presented here. Here we describe new structures of the stapes, especially in its proximal portion, as well as its position to the fenestra vestibuli. These structures are described for the first time not only in D. absitus, but for the genus.