末端脱氧核苷酸转移酶（Terminal deoxynucleotidyl transferase、TdT、末端转移酶），是一种特殊的DNA聚合酶，显示出未成熟的、"前B"，"前T淋巴细胞"和"急性淋巴性白血病/淋巴瘤细胞"。"末端转移酶"于"抗体基因重组时"加上N核苷酸（N-nucleotide）至V(D)J的外显子上，能够形成连接多样性（Junctional diversity）的现象。在人类中，末端转移酶是由DNTT基因来编码。

TdT不存在于"胎肝造血干细胞"上，而在胎儿期明显的损害B细胞之连接多样性。

Terminal deoxynucleotidyl transferase (TdT), also known as DNA nucleotidylexotransferase (DNTT) or terminal transferase, is a specialized DNA polymerase expressed in immature, pre-B, pre-T lymphoid cells, and acute lymphoblastic leukemia/lymphoma cells. TdT adds N-nucleotides to the V,D, and J exons of the TCR and BCR genes during antibody gene recombination, enabling the phenomenon of junctional diversity. In humans, terminal transferase is encoded by the DNTT gene. As a member of the X family of DNA polymerase enzymes, it works in conjunction with polymerase λ and polymerase μ, both of which belong to the same X family of polymerase enzymes. The diversity introduced by TdT has played an important role in the evolution of the vertebrate immune system, significantly increasing the variety of antigen receptors that a cell is equipped with to fight pathogens. Studies using TdT knockout mice have found drastic reductions (10-fold) in T-Cell receptor (TCR) diversity compared with that of normal, or wild-type, systems. The greater diversity of TCRs that an organism is equipped with leads to greater resistance to infection.