Illacme plenipes is a siphonorhinid millipede found in the central region of the U.S. state of California.

One of three known species in the genus Illacme, it was first seen in 1926, but was not rediscovered until 2005, almost 80 years after its discovery, by Paul Marek, then a Ph.D. student at East Carolina University.

Females grow to just over 3 cm; males are slightly smaller and have fewer legs.

[6] Marek and colleagues produced a more detailed description of the morphology of I. plenipes in 2012 and provided refined illustrations based on scanning electron micrography.

[7] Based on a phylogenomic analysis of millipedes in the subterclass Colobognatha, the closest relative of Illacme is Nematozonium of South Africa which shares a long, narrow body shape and characteristics including gonopods.