Recent research has documented widespread biofluorescence across amphibians. Among caudates in particular, representative species from 8 of the 10 families fluoresce under blue light excitation. Although fluorescence has been reported on the ventral surface of the tail in Eastern Red-backed Salamanders, Plethodon cinereus, nothing is known about the source or function of the fluorescence. This fully terrestrial salamander species has a broad geographic range, has complex mating behavior involving chemosensory and visual behaviors, defends territories from conspecific and heterospecific intruders, and is a model organism for studying many aspects of sociobiology. Our goal was to learn more about the source of fluorescence in P. cinereus and to explore demographic patterns of fluorescence. Additionally, we wanted to establish whether there is seasonal and geographic variation in fluorescence in this widespread salamander species. Through our examination of histological literature and close examination of photos of the ventral surface of tails under both white and UV light, we concluded that S1 glands, which comprise the postcloacal gland (PCG) in P. cinereus, are the source of fluorescence on the ventral portion of the tail. We found this trait to be highly sexually dimorphic, with males having significantly more fluorescent S1 glands both across seasons and localities compared to females. Additionally, we found that fluorescence only occurs in adult salamanders. Further, because the ventral surface of the tail is exposed during courtship and during territorial disputes, we hypothesize that male fluorescence of the PCG may function in the context of both mate choice and territoriality.