Urbanization is a major driver of environmental change that shapes the evolution of populations. However, the effects of environmental differences among cities on neutral and adaptive evolution remains poorly understood. We investigated evolutionary patterns in Impatiens capensis, a native wildflower that can be found in many cities in eastern North America. We used genotype-by-sequencing to evaluate genetic variation, contemporary demographic history, and local adaptation across 10 cities in Ontario, Canada. Urbanization and city size shaped the amount of genetic diversity present at sites and contributed to fine-scale spatial genetic structure. We identified a signal of repeated population bottlenecks occurring across all cities that corresponded to the timing of rapid urban expansion in the region. City size was an important environmental predictor of local adaptation, highlighting the role of cities in driving the adaptive evolution of populations. Our findings provide one of the first examples of parallel demographic shifts in response to urbanization in plants and offer insights into why I. capensis may be particularly resilient to urbanization. Taken together, our results emphasize the role that urban parks can play in maintaining genetic diversity and facilitating adaptation, suggesting that prioritizing greenspace conservation is critical for maintain urban biodiversity.