The D arm is a feature in the tertiary structure of transfer RNA. It is comprised of the two D stems (four base pairs each each; 10-13 and 22-25) and the D loop. The D loop contains the base dihydrouracil. The D loop's main function is that of recognition. It is widely believed that it will act as a recognition site for aminoacyl-tRNA synthetase which is an enzyme involved in the aminoacylation of the tRNA molecule. The D stem is also believed to have a recognition role although this is yet to be proved.

It is a highly variable region and is notable for its unusual conformation due to the over-crowding on one of the guanosine residues. It appears to play a large role in the stabilization of the tRNA's tertiary structure.