Significance Parasitic plants are pests of many plants, including major crop species. An important step toward creating resistance to parasitic plants is gaining a better understanding of how these pathogens control the physiology and development of their hosts. We combined genetic, cell-biological, and biochemical methods to identify the plant hormone cytokinin as a mobile signal between the hemiparasitic plant Phtheirospermum japonicum and the host Arabidopsis thaliana . Transport of parasite-derived cytokinins induced morphological changes in host roots, revealing insights into how parasitic plants manipulate host development and laying the foundation for future explorations for bioactive molecule transfer from parasitic plants to hosts.