The fatlips minnow (Phenacobius crassilabrum) is a species of suckermouth minnow that was first described in the upper Tennessee River system. No fewer than 20 specimens were used for description, and the epithet crassilabrum (from the Latin crassus meaning thick or fat and labrum, or lip) was assigned to the species, separating it from Phenacobius catostomus, with which it had previously been identified. The first holotype specimem collected in 1947 was an adult female 8 millimetres (0.31 in) in length from the River.