Urbanization is globally gaining force and challenges biodiversity but has recently also emerged as an agent of evolutionary change. Seasonal phenology and life-cycle regulation are essential processes that urbanization is likely to alter through both the urban-heat-island effect (UHI) and artificial-light-at-night (ALAN). However, how UHI and ALAN affect the evolution of seasonal adaptations has received little attention. Here, we test for urban evolution of seasonal life-history plasticity, specifically changes in the photoperiodic induction of diapause in two lepidopterans, Pieris napi (Pieridae) and Chiasmia clathrata (Geometridae). We used long-term data from standardized monitoring and citizen science observation schemes to compare yearly phenological flight curves in six cities in Finland and Sweden to those of adjacent rural populations. This analysis showed for both species that flight seasons are longer and end later in most cities, suggesting a difference in timing of diapause induction. Then, we used common-garden experiments to test whether evolution of the photoperiodic reaction norm for diapause could explain these phenological changes for a subset of these cities. These experiments demonstrated a genetic shift for both species in urban areas towards a lower daylength threshold for direct development, consistent with predictions based on the UHI but not ALAN. The correspondence of this genetic change to the results of our larger-scale observational analysis of in situ flight phenology indicates that it may be widespread. These findings suggest that seasonal life-cycle regulation evolves in urban ectotherms and may contribute to eco-evolutionary dynamics in cities.