Two endocasts of the extinct marsupial Thylacosmilus atrox are studied. The specimens were found in outcrops at present considered of Pliocene Age. The followings are the conclusions arrived in the study: 1) The encephalon of Thylacosmilus atrox shows a well developed neocortex, with a deep rhinal fissure and the major marsupial neocortical sulci well observable. The homologies of the sulci was certainly easy in the major part of them, and consequently, functional inferencies were possible. 2) The somatic neocortical area shows certain distinctive characteristics. It appears that the size proportions between the mandibular and maxillary areas is reversed in T. atrox, which appears congruent with the infered function of its peculiar cranial anatomy. 3) The relative brain size of T. atrox is very heigh for a Polyprotodont marsupial being its encephalization quotients and progression indexes at the level of the living Diprotodonta. 4) Thylacosmilus atrox was a very peculiar extinct carnivore marsupial, which at the Late Tertiary in South America reached higher levels of encephalization than the posterior or living representatives of its same Order.