Populations of arctic-alpine plant species inhabit a range of environments, to which their life-history characters are expected to have adapted; yet geographic differentiation among populations is seldom studied. Brief growing seasons mean that germination characters are critical fitness traits, especially for annual species. In this study, striking differences in germination traits among three geographically distinct populations of the widely distributed arctic-alpine annual Koenigia islandica are found. The seeds (achenes) of the Colorado population, which experience the lowest summer temperatures, are conditionally dormant; only scarification breaks dormancy. This germination pattern is consistent in all four investigated subpopulations from Colorado, with no significant differences among them. In contrast, seeds originating in the Yukon, which experience extreme winter temperatures but a relatively warm summer, germinate readily after cold stratification, a pattern consistent with that of summer annuals. Seeds from the Norway population, which experience the mildest climate, germinate readily even if untreated. Cold stratification decreases germination fraction in the Norway population, a pattern characteristic of winter annuals. The strong population differentiation found here provides evidence for divergent selection operating among arctic-alpine habitats and suggests that further investigation of its adaptive significance is merited.