An allograft is a transplanted organ or tissue from a genetically non-identical member of the same species. Most human tissue and organ transplants are allografts.

In contrast, a transplanted organ or tissue from a genetically identical donor, i.e. an identical twin, is called an isograft, while a transplant from another species is termed a xenograft. When a tissue is transplanted from one site to another on the same patient, such as a skin graft or a tissue flap, it is termed an autograft.

Allografts and xenografts will be recognised by the recipient's immune system as foreign and will therefore be attacked in a process termed rejection; this does not occur in autografts or true isografts (although in practice, transplants between identical twins are usually covered with immunosuppressants in case they are not 100% genetically identical).