Species capable of regenerating lost body parts occur throughout the animal kingdom, yet close relatives are often regeneration incompetent. Why in the face of 'survival of the fittest' some animals regenerate but others do not remains a fascinating question. Planarian flatworms are well known and studied for their ability to regenerate from minute tissue pieces, yet species with limited regeneration abilities have been described even amongst planarians. Here we report the characterization of the regeneration defect in the planarian Dendrocoelum lacteum and its successful rescue. Tissue fragments cut from the posterior half of the body of this species are unable to regenerate a head and ultimately die. We find that this defect originates during the early stages of head specification, which require inhibition of canonical Wnt signalling in other planarian species. Notably, RNA interference (RNAi)-mediated knockdown of Dlac-β-catenin-1, the Wnt signal transducer, restored the regeneration of fully functional heads on tail pieces, rescuing D. lacteum's regeneration defect. Our results demonstrate the utility of comparative studies towards the reactivation of regenerative abilities in regeneration-deficient animals. Furthermore, the availability of D. lacteum as a regeneration-impaired planarian model species provides a first step towards elucidating the evolutionary mechanisms that ultimately determine why some animals regenerate and others do not.