The 26S proteasome is a multisubunit protease responsible for regulated proteolysis in eukaryotic cells. It is composed of one catalytic 20S proteasome and two 19S regulatory particles attached on both ends of 20S proteasomes. Here, we describe the identification of Adrm1 as a novel proteasome interacting protein in mammalian cells. Although the overall sequence of Adrm1 has weak homology with the yeast Rpn13, the amino- and carboxyl-terminal regions exhibit significant homology. Therefore, we designated it as hRpn13. hRpn13 interacts with a base subunit Rpn2 via its amino-terminus. The majority of 26S proteasomes contain hRpn13, but a portion of them does not, indicating that hRpn13 is not an integral subunit. Intriguingly, we found that hRpn13 recruits UCH37, a deubiquitinating enzyme known to associate with 26 proteasomes. The carboxyl-terminal regions containing KEKE motifs of both hRpn13 and UCH37 are involved in their physical interaction. Knockdown of hRpn13 caused no obvious proteolytic defect but loss of UCH37 proteins and decrease in deubiquitinating activity of 26S proteasomes. Our results indicate that hRpn13 is essential for the activity of UCH37.