The southern mouse-colored tyrannulet (Nesotriccus murinus) is a species of bird in subfamily Elaeniinae of family Tyrannidae, the tyrant flycatchers.

The International Ornithological Committee (IOC) and the Clements taxonomy treat the northern and southern taxa as separate species.

Their wings are dusky with wide whitish to dull cinnamon edges on the innermost flight feathers and tips on the coverts.

Subspecies N. m. wagae has less white on the throat, a darker green shade to its upperparts, and more intense yellow underparts than the nominate.

Both sexes of both subspecies have a brown iris, a thick, rounded, horn-colored bill with pale pink at the base of the mandible, and gray legs and feet.

[2] Clements places it similarly but adds northwestern Argentina and instead of including the Guianas says its "northern distributional limit not certain, may occur farther north in eastern South America".

[3] Cornell University's Birds of the World places it in Brazil, Bolivia, and Paraguay like the other two but not in the Guianas, and also includes northwestern and northeastern Argentina.

[2][3][5] The southern mouse-colored tyrannulet inhabits a variety of open to semi-open landscapes, most of which are arid to only moderately humid.

As best is known, it primarily feeds on insects though fruits of mistletoe (Loranthaceae) and some other plants are a significant part of its diet.

The female alone builds the nest, an open cup of plant fibers and roots, grasses, mosses, and spider web with feathers in the structure and as a lining.

It is typically placed in a branch fork or tree crotch within about 4 m (13 ft) of the ground, though occasionally higher.