Akodon caenosus is very small, averaging 19.3 g (0.68 oz) in weight, and variable in coloration, but generally brown.

The skull has a short rostrum (front part), broad interorbital region (between the eyes), and narrow braincase.

[9] In 1920, Thomas recognized additional differences between the two after examining more specimens and classified the Argentine form as a separate species, Akodon cænosus.

[12] They retained caenosus as a separate subspecific name for the Argentine populations of puer because of its small size, dark fur,[13] and distinctive karyotype.

[15] In 1999, Mónica Díaz and others described these animals more fully as a new species, Akodon aliquantulus, which they considered closely related to A. puer caenosus.

[17] In the 2005 third edition of Mammal Species of the World, Guy Musser and Michael Carleton termed the differentiation between A. aliquantulus and A. lutescens (=puer) "unimpressive" and recommended further taxonomic research.

[25] The upperparts are uniformly colored, but their tone is variable: generally ochraceous brown, but approaching yellow, red, or olivaceous in some individuals.

[26] In the skull, the rostrum (front part) is short, the interorbital region (between the eyes) is broad and hourglass-shaped, and the braincase is small.

In the mandible (lower jaw), the masseteric ridges, which anchor some of the chewing muscles, extend to near the front margin of the first molar.

The capsular process, raising in the back part of the mandibular bone that accommodates the root of the incisor, is poorly developed.

[14] Members of the Akodon boliviensis group, including A. caenosus, are generally similar and difficult to separate,[27] but they differ in relative cranial measurements and some other characters.

It occurs together with A. boliviensis, A. sylvanus, A. simulator, and species of Oxymycterus, Calomys, Phyllotis, Oligoryzomys, Necromys, Andinomys, Graomys, and Abrothrix.