Beyond the observation of climatic variations and their impact on livelihoods, farmers' knowledge about climate change can help to understand how rural populations respond to environmental changes and what factors should be considered when planning rural adaptation. This study documents Sereer farmers' observations of local environmental changes in the Fatick region of Senegal and explores how the farmers use crop diversity to adapt to those changes. Their observations of environmental changes were documented through focus group discussions and semi-structured interviews. Variations in crop diversity, as well as farmers' explanations for these variations, were assessed through surveys in two villages (n = 126 households). Sereer farmers identify four distinct periods of similar climate trends and reported how they managed crop diversity in response to the climate variations between periods. Three management responses stand out: 1) abandonment of long-cycle varieties during drought periods, 2) adoption of short-cycle varieties during periods with shorter rainy seasons, and 3) reinstating of long-cycle varieties with the return of rains. Sereer farmers consider that climate variations are important reasons to modify their crop varieties, although variety selection is also affected by other socio-economic and cultural reasons. This study illustrates the contributions that local knowledge can bring to understanding the local impact of climate change on smallholder farmers. Understanding how they use crop diversity to adapt to climate variations can be the basis of climate change adaptation policies that address local needs and constraints.