Adult males are slightly smaller than the females, ranging from 58-91 mm in total length and averaging 73 mm. Adult females range from 64-90 mm and average 78 mm. The largest individual on record is 122 mm (Bishop 1943).
The red-backed phase of this species is characterized by a broad, dorsal band running down the midline from the head onto the tail. The color of the stripe varies from light gray or dull yellow to pink, brick-red, and bright red. There are often small flecks of black within the band. The sides are dark gray or black, becoming lighter and mottled toward the belly, which is strongly mottled with white and gray. In contrast, the lead-backed phase lacks the dorsal band and is uniformly dark gray to almost black, with the head and legs usually lighter (Bishop 1943). There is also an erythristic color phase that is mostly red, apparently to mimic juvenile Notophthalmus viridescens (Tilley et al. 1982). Juveniles of the red-backed phase have a well developed dorsal band and the upper sides are strongly pigmented (Bishop 1943)
The body is long and fairly slender, is slightly flattened dorsally, and is well rounded on the sides. The cross section of the tail is nearly circular throughout its length. Regenerating tails are flattened laterally and are usually uniform dark gray. Number of costal grooves ranges from 17 to 20, but there are usually 18 or 19. The gular fold is prominent. The legs are small with short, thick toes. There are four fingers, which in order from longest to shortest are 3-2-4-1. The five toes are slightly webbed, and are 3-4-2-5-1 in order from longest to shortest. The vomerine teeth form two backward-curving lines of 5-7 teeth separated from each other and from the parasphenoid teeth, which are in two imperfectly separated patches. The mouth is fairly large, with the angle of the jaw behind the eye. The small tongue does not fill the floor of the mouth. Males can be identified when in breeding condition by swollen snout, enlarged premaxillary teeth, and proportionally longer legs (Bishop 1943). Black testes can also be seen through the abdominal wall when transiluminated by a strong light (Jaeger et al. 2002a).
As is the case for all members of the genus Plethodon, eggs are laid in terrestrial cavities attended by the female. The larval stage is passed within the egg capsule. The broad, flat, leaf-like gills rise from a common base, are often fully developed at hatching, and then persist for only a few days (Bishop 1943). Embryos average about 19 mm upon hatching and individuals less than 32 mm in snout-vent length are considered to be juveniles (Bishop 1943; Jaeger et al. 2002a). Juveniles have proportionately broad heads, which allows them to forage on a wide range of prey (Maglia 1996). The fingers and toes of the juveniles are well indicated, the inner and outer short (Bishop 1943).
While populations from the formerly glaciated part of the range are very uniform, allozyme studies show that when its entire range is considered, P. cinereus consists of four genetically differentiated geographic groups with within-group D-values ranging from 0-0.15 and between-group D-values ranging from 0.02-0.24. This indicates that the groups living in the unglaciated localities have been isolated from each other for 1.5-2.7 million years, and that populations from formerly glaciated areas are all descended from the same group. Despite their long divergence, there is still extensive gene flow between the groups at the points where they contact one another (Highton and Webster 1976; Highton 2000).