Illacme plenipes is a siphonorhinid millipede found in the central region of the U.S. state of California. It has up to 750 legs. 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.
On average, I. plenipes have over 600 legs, twice the average for millipede species, with one recorded specimen having 750 legs. It had the most legs of any species known until Eumillipes persephone was described in 2021, which had 1306 legs. It is relatively small-bodied among millipedes. Females grow to just over 3 cm; males are slightly smaller and have fewer legs.