DNA polymerase zeta (Polzeta) is an error-prone DNA polymerase involved in translesion DNA synthesis. Polzeta consists of two subunits: the catalytic REV3, which belongs to B family DNA polymerase, and the noncatalytic REV7. REV7 also interacts with REV1 polymerase, which is an error-prone Y family DNA polymerase and is also involved in translesion DNA synthesis. Cells deficient in one of the three REV proteins and those deficient in all three proteins show similar phenotype, indicating the functional collaboration of the three REV proteins. REV7 interacts with both REV3 and REV1 polymerases, but the structure of REV7 or REV3, as well as the structural and functional basis of the REV1-REV7 and REV3-REV7 interactions, remains unknown. Here we show the first crystal structure of human REV7 in complex with a fragment of human REV3 polymerase (residues 1847-1898) and reveal the mechanism underlying REV7-REV3 interaction. The structure indicates that the interaction between REV7 and REV3 creates a structural interface for REV1 binding. Furthermore, we show that the REV7-mediated interactions are responsible for DNA damage tolerance. Our results highlight the function of REV7 as an adapter protein to recruit Polzeta to a lesion site. REV7 is alternatively called MAD2B or MAD2L2 and also involved in various cellular functions such as signal transduction and cell cycle regulation. Our results will provide a general structural basis for understanding the REV7 interaction.