Adults are 100-230 mm TL and have 17-19 costal grooves (Stuart et al. 2008). Most individuals do not exceed 100 mm SVL (Dent and Kirby-Smith 1963). This species has a stout body, reduced black eyes, a broad head, and a truncated snout (Petranka 1998). Its gills are long, bright red, and tend to curl upward and forward over the head (McCrady 1954). The tail is oar-like and compressed laterally (McCrady 1954). The skin is generally translucent and sparsely pigmented, appearing pinkish to flesh-colored from underlying blood capillaries, depending on the subspecies (G. p. palleucus is pale while G. p. necturoides is dark; Lazell and Brandon 1962). There may also be tannish-gray pigmentation and spots dorsally and laterally (McCrady 1954).

This species is mostly neotenic, but naturally metamorphosed individuals are occasionally found (Yeatman and Miller 1985, Sinking Cove Cave; Brandon et al. 1986, Custard Hollow Cave). It can be induced to metamorphose in a laboratory setting; Dent and Kirby-Smith (1963) reported that two untreated individuals metamorphosed completely and one metamorphosed partially, while Blair (1961) reported that one individual metamorphosed after three months of iodine-potassium iodide treatment but another did not metamorphose after six months of treatment, and Yeatman (1967) was also able to hormonally induce metamorphosis in this species.

Gyrinophilus palleucus has 2 subspecies, G. p. necturoides and G. p. palleucus. Subspecies differ slightly in body pigmentation, head width, leg length, eye size, and modal number of trunk vertebrae (Brandon 1966; Brandon 1967).

The Big Mouth Salamander, G. p. necturoides, differs from G. p. palleucus by having one or two additional costal grooves (usually 18 costal grooves, sometimes 19 in G. p. necturoides; usually 17 costal grooves, sometimes 18 in G. p. palleucus) and by its darker, spotted coloration. G. p. necturoides ranges in color from a russet brown to dark brown to deep purple-brown on the dorsum, with numerous blackish spots that extend from the level of the jaw down to the anterior third of the tail. A dark chevron-like pattern is sometimes present. The venter is pearl-gray, with rosy coloration at the limb insertions and the vent. Two parallel lateral rows of small white dots, corresponding to the costal spaces, run along the sides from the axilla onto the basal half of the tail. A distinct, pale cheek stripe extends from the base of the most ventral gill ramus onto the side of the head. Gills are a deep brilliant red and extend back to the fourth costal groove. Between adpressed limbs, 7-8 costal grooves are visible (Lazell and Brandon 1962).

The Pale Salamander, G. p. palleucus, is the paler of the 2 subspecies and generally has fewer costal grooves (17 costal grooves, sometimes 18 in G. p. palleucus, vs. 18 costal grooves, sometimes 19 in G. p. necturoides; Lazell and Brandon 1962).
 
The Tennessee Cave Salamander complex includes G. palleucus (with two subspecies, G. p. palleucus and G. p. necturoides) and G. gulolineatus, the Berry Cave Salamander (Miller and Niemiller 2008). Some populations in caves of the Tennessee Valley in northeastern Alabama (Blowing Cave, Jackson Co.) may be intergrades between G. p. palleucus and G. p. necturoides, based on the coloration and spotting pattern (Lazell and Brandon 1962; Brandon 1965; Brandon 1966; Brandon 1967).