Middle Permian (Guadalupian) strata containing brachiopods are present in both the accreted terranes and continental margin areas of western North America. Though not as diverse and abundant there as the Early Permian faunas they replaced, Middle Permian brachiopods still provide important data for paleobiogeography, with implications for paleogeography. A database of 86 brachiopod genera from 16 localities was generated from both the published literature and as yet unpublished data. Terrane localities include (present-day north to south): Cache Creek, Quesnellia, Wallowa, Eastern Klamath, Black Rock (3 separate sub-localities), Northern Sierra and El Antimonio. Continental Margin localities/formations include (north to south): Arctic Canada, Tahkandit, Ranger Canyon, Park City, Edna Mountain, Kaibab and Owens Valley. The probabilistic similarity algorithm of Raup & Crick (1973) tested the null hypothesis that the brachiopod genera are randomly sprinkled over all sampled localities. Over 10,000 random simulations were run with the following results: 1) most relationships between any two localities were not significant at the 95% level as predicted by the algorithm (3.91% positive similarity, 7.03% negative similarity); 2) positive similarity pairs between the Black Rock sub-localities affirming terrane cohesion; 3) positive similarity between Wallowa and Northern Sierra terranes, supporting previous hypotheses that these two are part of the same terrane; 4) positive similarity between Eastern Klamath and Kaibab, and between Quesnellia and Arctic Canada, both probably due to paleogeographic proximity; 5) negative similarity between Park City and Quesnellia, Eastern Klamath and El Antimonio, all possibly explained by the unique facies of the Park City; 6) negative similarity between Tahkandit and Black Rock, N. Sierra, El Antimonio and Owens Valley; and between Quesnellia and Owens Valley, all of which support paleogeographic separation. No other paleobiogeographic relationships have statistical significance, and can have no implications for paleogeography.