The latitudinal gradient in body mass, which many organisms show (Bergmann's rule), is thought to be a response to local environmental conditions varying across latitude. Hence, temporal changes in environmental conditions are expected to lead to a shift in Bergmann's rule. We compared house sparrow Passer domesticus body mass measures taken in 1984/85 with measures in 2009 to show that the mean body mass decreased in eight and nine populations (for females and males, respectively) out of ten Finnish populations. Consequently, the latitudinal cline in body mass shifted poleward. The decrease in body mass was large (1.5 g reduction in a 33-g bird). Temperatures during sampling did not differ between sampling periods, suggesting that at least the immediate effect of temperature did not explain the reduction in body mass. House sparrows have declined in abundance in Finland and worldwide in recent decades. We here suggest that the deterioration of the (unknown) environmental conditions associated with this population decline (which may include climatic drivers) has led to a poleward shift in Bergmann's rule in house sparrows.