The species of the genus Spilomicrus in America north of Mexico are revised. A generic diagnosis of Spilomicrus, key to genera of Nearctic Spilomicrini, and key to Spilomicrus species in America north of Mexico are given. Four species-groups are defined, viz. distinctus, exul, formosus, and stigmaticalis. Character states, variation, distribution, and relationships of each species are discussed. Twelve of 21 species are recognized as new: S. boweni, S. crassicornis, S. distinctus, S. exul, S. inornatus, S. palustris, S. pumilio, S. robustus, S. sanbornei, S. stepheni, S. superbus, and S. sylvicola. Nine previously known species are redescribed: S. antennatus (Jurine), S. atriclavus Ashmead, S. bifoveatus Kieffer, S. brevicornis (Ashmead), S. flavicornis Ashmead, S. formosus Jansson, S. ruficornis (Provancher), S. stigmaticalis Westwood, and S. virginicus Fouts. The males of S. brevicornis and S. virginicus, as well as the females of S. flavicornis and S. ruficornis, are described for the first time. The following new synonymies are proposed: S. antennatus (= Basalys californica Ashmead), S. atriclavus (= S. foutsi Masner, S. kiefferi Fouts), and S. stigmaticalis (= S. armatus Ashmead).