Significance Retroviral envelope gene capture and exaptation for a placental function has been demonstrated in mammals. Remarkably, placental structures have also emerged on rare occasions in nonmammalian vertebrates, resulting in related modes of reproduction. The Mabuya lizard, which emerged 25 Mya, possesses a placenta closely related to that of mammals. Here, we identified a specific retroviral envelope gene capture that shows all the characteristic features of a bona fide mammalian syncytin, being conserved in Mabuya evolution, expressed in the placenta, and fusogenic. Together with the present identification of its cognate receptor, these results show that syncytin capture is not restricted to mammals and is likely to be a major driving force for placenta emergence.