<p>A diverse urban forest increases resilience to abiotic and biotic disturbances. Previous research has examined regional taxonomic diversity, but few studies have explored national-level urban forest diversity. Using open street tree inventory data from 32 Canadian cities, we identified the most abundant street tree species, genera, and families and examined the influence of population and environmental factors on diversity measures and native species dominance. We found that maple (<i>Acer</i>), ash (<i>Fraxinus</i>) and elm (<i>Ulmus</i>) account for nearly half (46%) of all street trees. Four species – Norway maple (<i>Acer platanoides</i>), green ash (<i>Fraxinus pennsylvanica</i>), American elm (<i>Ulmus americana</i>), and thorny honey locust (<i>Gleditsia triacanthos</i>) – comprise more than one third (34%) of all street trees. Relative abundance of the most dominant species, genus, or family was highest in smaller cities. Diversity and evenness indices were most often explained by the average number of frost-free days per year and, commonly at the species level, average annual precipitation. Diversity but not evenness was greater outside of downtown areas. The proportion of native trees in street tree populations varied widely, ranging from 4.6 to 89.2%. Native proportion was greater in colder, harsher climates, but comparisons between neighboring cities reveal that urban forest practices and management decisions are also likely influential. A nearly equal number of cities had a greater proportion of native street trees downtown as had fewer native street trees downtown. The results provide new insight into urban tree biogeography and underscore the need for continued diversification of Canadian urban forests.</p>