Caecilia mertensi is a species of caecilian in the family Caeciliidae from South America. The type locality is imprecise: the holotype was purportedly collected in "Seychelle Isle", but most likely originates from South America. However, more recently, several specimens have been collected from Mato Grosso, Brazil. The specific name mertensi honors Robert Mertens, a German zoologist and herpetologist. Common name Mertens' caecilian has been coined for this species.
The holotype is unsexed and measures 495 mm (19.5 in) in total length. The body is 12.5 mm (0.5 in) wide. The head is slightly tapering, and the snout is projecting. The eyes are very small but visible externally. There are 142 mostly incomplete primary folds and 48 secondary folds. The body is partly covered by scales. Coloration is light brown, turning into grayish brown laterally. The head is grayish.
