Coral bleaching, a major threat to reef ecosystems, involves the breakdown of the symbiosis between coral hosts and their photosynthetic endosymbionts (Symbiodiniaceae). To characterize the cellular and molecular changes underlying bleaching at single-cell resolution, we generated a single-cell RNA-sequencing atlas of the coral Acropora cervicornis during a thermally-induced bleaching experiment. Our atlas captures diverse host cell types, including immune, digestive, and endodermal cells, as well as distinct Symbiodiniaceae cell states. We identify key transcriptional signatures associated with bleaching stress, including dysregulation of symbiosis-maintenance pathways, activation of immune responses, and metabolic shifts in both host and symbiont cells.