Regeneration requires functional coordination of stem cells, their progeny, and differentiated cells. Past studies have focused on regulation of stem cell identity and proliferation near to the wound-site, but less is known about contributions made by differentiated cells distant to the injury. Here, we present a comprehensive atlas of whole-body regeneration over time and identify rare, transient, somatic cell states induced by injury and required for regeneration. To characterize amputation-specific signaling across a whole animal, 299,998 single-cell transcriptomes were captured from planarian tissue fragments competent and incompetent to regenerate. Amputation-specific cell states were rare, non-uniformly distributed across tissues, and particularly enriched in muscle (mesoderm), epidermis (ectoderm), and intestine (endoderm). Moreover, RNAi-mediated knockdown of genes up-regulated in amputation-specific cell states drastically reduced regenerative capacity. These results identify novel cell states and molecules required for whole-body regeneration and indicate that regenerative capacity requires transcriptional plasticity in a rare subset of differentiated cells.