Arctic fauna is undergoing significant alteration in response to global climate change, yet we know little regarding the factors that determine species assemblages at northern latitudes. We used a latitudinal transect to assess environmental determinants of ground-dwelling spider assemblages across the boreal forest-tundra transition at a regional scale. Using multivariate techniques, we tested 3 complementary hypotheses regarding the factors that best explain patterns of assemblage structure. We predicted that spider assemblages would respond most strongly to vegetation composition and structure and that climate and spatial variables would explain less of the variation in the data. We sampled ground dwelling spiders using pitfall traps placed at 36 sites along the latitudinal transect. We constructed 3 separate matrices of spatial, climate, and vegetation variables, with each matrix representing a hypothesis. We used redundancy analysis with variation partitioning to determine which matrix of environmental variables best explained patterns in a matrix of spider abundances. We then used a separate redundancy analysis to determine which environmental variables best explained the variation in measures of species richness and activity density. We collected a total of 2890 individual spiders representing 103 species, 58 genera, and 13 families. Our analysis supports the hypothesis that vegetation composition and its related structure best explain patterns in northern spider assemblages at a regional scale.