Pseudoeurycea mystax is a species of salamander in the family Plethodontidae. It is endemic to Mexico and only known from the area of its type locality near Ayutla, Oaxaca. Its common name is mustache false brook salamander or mustached false brook salamander. The specific name refers to the whitish protuberances on the lips that resemble a mustache in the frontal view of the male holotype.
The type series consists of two specimens, the male holotype measuring 47.5 mm (1.87 in) and the female allotype 42 mm (1.7 in) in snout–vent length. The tail is shorter than the body, with total lengths of about 85 and 69 mm (3.3 and 2.7 in) for the two specimens, respectively. The limbs are moderately short and stout. The dorsum is gray in its ground color with faint traces of pink especially on the snout, and with scattered black dots. The tail has some conspicuous, large white blotches.
