The timing of rains, and intra-seasonal rainfall patterns, are critical to smallholder farmers in developing countries. Seasonality influences farmer’s decisions about when to cultivate and sow and harvest, and ultimately the success or failure of their crops. Worryingly, therefore, farmers are reporting that both the timing of rainy seasons and the pattern of rains within seasons are changing. These perceptions of change are striking in that they are geographically widespread and because the changes are described in remarkably consistent terms. In this paper, we relate the perceptions of farmers from several regions (East Asia, South Asia, Southern and East Africa and Latin America) of how seasons are changing, and in some cases, how once distinct seasons appear to be disappearing altogether, and the impacts that these changes are having. We then go on to ask two critical questions. Firstly, do meteorological observations support farmers’ perceptions of changing seasonality? Secondly, to what extent are these changes consistent with predictions from climate models? We conclude that changing seasonality may be one of the major impacts of climate change faced by smallholder farmers in developing countries over the next few decades, and indeed, may already be so. Yet it is relatively unexplored in the literature. We also suggest some of the key adaptation responses that might help farmers cope with these changes.