In northwestern North America, the milliped family Nearctodesmidae is comprised of four genera, Nearctodesmus Silvestri, Kepolydesmus Chamberlin, Ergodesmus Chamberlin, and Bistolodesmus, new genus, and six species, N. insulans (Chamberlin), N. cerasinus (Wood), N. salix Chamberlin, K. anderisus Chamberlin, E. compactus Chamberlin, and B. bonikus (Chamberlin). These species occur along the Pacific coast from San Francisco Bay to Prince Rupert, British Columbia, including all offshore island groups, with an eastward extension into Lewis and Clark County, Montana, east of the Continental Divide. An allopatric population of N. insulanus occurs in the Shuswap Highlands of British Columbia, and an allopatric species, E. remingtoni (Hoffman), inhabits caves in western and southern Illinois. The principal taxonomic characters are the number of secondary projections from the gonopodal telopodite, the configuration of the longer of these, and the length and configuration of the distal zone of the acropodite. Polydesmus bonikus is a nearctodesmid and is assigned to the new genus, Bistolodesmus. The following new synonymies are proposed: Jaliscodesmus Hoffman under Sakophallus Chamberlin; J. alticola Hoffman under S. simplex Chamberlin; N. brunnior, N. campicolens, and N. malkini, all by Chamberlin, under N. cerasinus; N. amissus, N. pseustes, N. renigens, N. carli, and N. boydi, all by Chamberlin, and N. olympus and N. cochlearius, both by Causey, under N. insulanus; K. mimus, K. hesperus, and K. pungo, all by Chamberlin, under K. anderisus; and Ectopodesmus cristatus and E. c. dentatus, both by Loomis and Schmitt, under Ergodesmus compactus. Modern diagnoses and illustrations are presented for the family and all northwestern taxa, along with keys to the genera and species of Nearctodesmus. Accounts and gonopod drawings are also presented of Sakophallus and S. simplex Chamberlin in Michoacan and Jalisco, Mexico.