Significance Global change threatens the genetic diversity of economically important and foundational ecosystem-building species such as corals. We tested whether cryopreserved coral sperm could be used to transfer genetic diversity among genetically isolated populations of the critically endangered Caribbean elkhorn coral, Acropora palmata . Here we report successful assisted gene flow (AGF) in corals using cryopreserved sperm, yielding the largest living wildlife population ever created from cryopreserved cells. Furthermore, we produced direct evidence that genetically distinct populations of Caribbean coral can interbreed. Thus, we demonstrated that sperm cryopreservation can enable efficient, large-scale AGF in corals. This form of assisted genetic migration can enhance genetic diversity and help critically endangered species adapt to local environments in the face of rapid global change.