The North American fauna of the Laurasian family Sironidae is examined phylogenetically and compared with species from Europe and Japan. The North American clade is not resolved as monophyletic. The phylogenetic analyses and detailed morphological study identified four cryptic species of sironids in the western United States, formerly considered within the geographical and morphological range of Siro acaroides (Ewing, 1923). These four species are described as Siro boyerae sp. nov., Siro calaveras sp. nov., Siro clousi sp. nov., and Siro shasta sp. nov. We also provide new localities for the previously known species in the western United States. Siro boyerae sp. nov. forms a clade with Siro kamiakensis (Newell, 1943) and with the East Coast species Siro exilis Hoffman, 1963, characterized by the presence of narrow coxae III that do not meet along the midline. The affinities of S. calaveras sp. nov., S. clousi sp. nov., and S. shasta sp. nov. remain largely unresolved, but S. clousi sp. nov., is not related to S. acaroides despite being found sympatrically.