The Chacoan pygmy opossum (Chacodelphys formosa) is a recently described genus and species of didelphimorph marsupial.

The Chacoan pygmy opossum is the smallest known species of didelphid.

It differs from the other "marmosine" genera (Marmosa, Monodelphis, Thylamys, Tlacuatzin, Gracilinanus, Marmosops, Lestodelphys) in having a long third manual digit, no distinctly tricolored pelage, a long fourth pedal digit, and a tail shorter than head-body.

Afterwards, George Tate (1933) considered it a valid member of his "Elegans group" (=Thylamys) of Marmosa, whereafter it has been variously synonymized or treated as a distinct species of Marmosa or Thylamys until 1989, when Gardner & Creighton (1989) listed it as a synonym of Gracilinanus agilis, and then later separated from this species as G.

[2] Finally, Voss et al. (2004) erected the new genus Chacodelphys for the species.