While temperature controls on vegetation phenology in humid temperate climates have been widely investigated, water availability is the primary limit on vegetation growth in arid and semiarid ecosystems at continental and global scales. This paper explores the response of vegetation phenology to precipitation across Africa from 2000 to 2003 using vegetation index data from the Moderate-Resolution Imaging Spectroradiometer (MODIS) and daily rainfall data obtained from the Tropical Rainfall Measuring Mission (TRMM). The results indicate that well-defined thresholds exist in cumulative rainfall that stimulate vegetation green-up in arid and semiarid regions of Africa. Therefore cumulative rainfall provides an appropriate criterion for determining the onset of the rainy season, which can be used to predict the onset of vegetation green-up. Following the end of the rainy season, the onset of vegetation dormancy occurs with a lag of about 54 and 84 days in the Sahelian and sub-Sahelian region and in southern Africa, respectively. Further, spatial patterns in vegetation phenology can be divided into three distinct regions in arid and semiarid ecosystems of Africa because of well-defined patterns in rainfall seasonality. In response to rainfall patterns controlled by the Intertropical Convergence Zone in the Sahelian and sub-Sahelian region, the timing of vegetation phenology shifts gradually in a north-south direction at a rate of 0.12 d/km for green-up onset and 0.05 d/km for dormancy onset. In contrast, patterns in vegetation phenology and rainfall seasonality are much more complex in southern Africa. The shift rates and trends in this region are consistent locally but vary abruptly across different ecosystems or climate regimes. Multiple annual cycles of vegetation growth closely follow rainy seasons in parts of the Greater Horn of Africa.